I'll address this one first, since it is the shortest answer.
I wonder how it's legal to charge a fee based on the age of the traveller?
The airline doesn't charge for baggage based on age; the OP's title is misleading, and your response reinforces that point. However, if the airline felt the luggage of a segment of the population was more difficult to handle, then they could likely charge for that. However, I doubt simply charging based on age would be defensible in court. In the end, good point, the airlines don't charge based on age of the passenger.
I don't care what they call the fee or why they say they charge it.
Semantics do matter, especially when a DISer attempts to find the same information using the board's search function. Poor titling leads to several problems such as passing misinformation and preventing others from gleaming good info from a discussion. I'm sorry you don't care, but it does matter.
I'm sorry you felt the need to try and belittle my credibility and knowledge in travel in order to elevate your position, even though there are no reinforcing facts to back up your claims other than "that's how others do it". I am no expert in travel, by any means; however, when I have a question, I know where to get the information. So, let us see what the airlines themselves say instead of how you feel they should operate...
We have flown other airlines this way for several years and at one point had both kids on a seperate flight than either of us! No problems... All I know is I can't book online myself due to a policy about children, I can't even say unaccomponied children because they will have a parent each way, it is thier system that does not allow for this to be verified anywhere and thier policy that requires it.
So, instead of railing on it, let's see what the airline actually says about it, shall we?
ACCEPTANCE OF CHILDREN
Accompanied
Children 2 through 11 years of age will be accepted for transportation when accompanied by a passenger at least 16 years of age.
Unaccompanied
A service charge will be assessed for unaccompanied children on all flights.
Unaccompanied children a child/minor under 12 years of age not accompanied on the same flight and in the same compartment by a passenger at least 16 years of age
Unaccompanied children under 5 years of age will not be accepted under any circumstances.
Unaccompanied children 5, 6 or 7 years of age will be accepted on a nonstop or through flight only, and must be accompanied by a parent or responsible adult until the child is boarded on the flight and the aircraft has departed the gate**. The child must be met at the destination by another parent or responsible adult.
** Note: Unaccompanied minors ages 5-7 are restricted on flights scheduled with a change from one aircraft to another type of aircraft before termination, but designated by a single flight number.
Unaccompanied children 5, 6, or 7 years of age will not be accepted on certain American flights operated in equipment not requiring a flight attendant. Contact American for specific information.
Unaccompanied children 8 through 11 years of age will be accepted for transportation on nonstop, through or connecting flights. Reservations must be confirmed to the destination. The child must be accompanied by a parent or responsible adult until the child has boarded the flight and the aircraft has departed the gate. The child must be met at the destination by another parent or responsible adult.
American does not accept unaccompanied children when their itinerary includes a connection to/from another airline, including codeshare and oneworld partners.
American will not accept reservations or provide transportation for unaccompanied children for any itinerary that includes the last online connecting flight of the day.
American may refuse to provide connection air transportation services at any origination city to an unaccompanied child holding reservations when there is a reasonable likelihood that the child will not make a flight connection, and therefore require overnight accommodations.
https://www.aa.com/i18n/customerService/customerCommitment/conditionsOfCarriage.jsp#AcceptChild
Above, I've emboldened the definition of an accompanied and unaccompanied minor. Ok, so the big question is; what ages are your children? As you will see, airlines have different rules on this. For AA, a minor of less than 12 years old may be accompanied by a person 16 years old or more. Otherwise, a special fee (unaccompanied minor) applies.
If the age of the passenger is 12 or less, then a passenger age 16 or more must be on the same itinerary (PNR) to book the ticket online, since the computer has no way of knowing the intricacies you propose. That is why the computer asks you to call a service rep for ticketing. Fair enough?
Now, lets look at "other airlines". I'm not sure who you've flown before (since you used the plural of this word), but lets have a look at Southwest (WN)...
Carriage of Children
(1) Accompanied Minor Children.
(i) Infants 14 days of age or younger. Carrier will not provide transportation services to any infant 14 days of age or younger, unless a written statement is provided by an attending physician approving such infant for air travel. Infants must be accompanied by a Passenger 12 years old or older.
(ii) Children 14 days through two years old. One child 14 days up to two years old on the date of travel may be carried on the lap of an accompanying Passenger 12 years of age or older. If an adjacent unoccupied seat is available, the child may be secured in an FAA-approved child restraint device without charge. However, if the child is traveling without a confirmed reservation and no adjacent
unoccupied seats are available, the child restraint device may have to be transported as Checked Baggage.
(iii) Children 14 days up to two years old traveling on a confirmed reservation, with or without the use of an FAA-approved child restraint device, will be charged the published Infant Fare (see Article 3) or lowest available adult fare, whichever is less.
(2) Unaccompanied Minor Children.
(i) Children younger than five years old. Carrier will not accept for Carriage any child less than five years old unless accompanied by a Passenger at least 12 years of age.
(ii) Children five through 11 years old. Unaccompanied children ages five through 11 years old will be required to use Carriers unaccompanied minor service and will be accepted for Carriage by Carrier provided the child has a confirmed reservation and the flight on which he or she travels does not require a change of aircraft or flight number. However, any unaccompanied child age five through 11 years old will not be accepted for Carriage if, because of operational disruptions, the Carrier determines, in its sole discretion, that the flight on which the child holds a reservation is anticipated to terminate short of or bypass the childs destination.
(v) Unaccompanied Minor Charge. In addition to the applicable fare, children for whom unaccompanied minor Carriage is required must pay the applicable unaccompanied minor charge. If travel does not take place, the charge is nonrefundable.
http://www.southwest.com/assets/pdfs/corporate-commitments/contract-of-carriage.pdf
So, it seems WN believes a minor to be a person under the age of 12 (not 16 like AA).
I just did a dummy booking on WN's website, and there is a disclaimer at the bottom that reads:
For infant, child (2-11), group (10+), and military fares please call 1-800-I-FLY-SWA (1-800-435-9792). These fares are a discount off the "Anytime" fares. Other fares may be lower.
Plus, WN online booking system considers ages 2+ as an adult, thus it is likely you may have been able to book online for your children, while you can not on AA's site.
As a final data point, let's see what United (UA) thinks about all of this...
Unaccompanied Minor means a Child/Minor 5 to 11 years of age when traveling alone or not accompanied on the same flight and in the same compartment by a companion Passenger
at least 18 years of age or with a Legal Guardian or parent.
B) Children/Minors Traveling Unaccompanied
1) UA offers Unaccompanied Minor service for children/minors under age 18 who are not accompanied by a passenger who is at least 18 years old or a Parent/Legal Guardian. The policies for UAs Unaccompanied Minor service apply only to flights operated by UA. See Rule 18 regarding travel on or connection to flights operated by a carrier other than UA. Customers should review the rules for flights operated by another carrier and familiarize themselves with the policies regarding Unaccompanied Minors, which may differ from UAs policies for Unaccompanied Minor service.
2) Unaccompanied children under five (5) years of age are not accepted on flights operated by UA and Carriers doing business as United Express.
3) UAs Unaccompanied Minor service is mandatory for unaccompanied children age five (5) to eleven (11) years old and optional for unaccompanied minors age 12 to 17 years old.
4) Unaccompanied children/minors are not accepted on flights operated by UA that connect to/from flights operated by a carrier other than UA, including other UA codeshare partners.
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5) Unaccompanied children age five (5) to seven (7) years old may only travel on nonstop or direct flights operated by UA and Carriers doing business as United Express.
6) Unaccompanied children age eight (8) to eleven (11) years old, and any other unaccompanied minors for whom the service fee is paid, may travel on nonstop, direct, and connecting flights operated by UA and Carriers doing business as United Express, except on the last connecting flight of the day to their final destination unless the connecting flight is the only published service to that destination. Unaccompanied children/minors are not accepted on flights which require an overnight stay in order to make a connection.
7) Unaccompanied children/minors must be brought to the airport of departure by a parent, legal guardian, or responsible adult who shall remain with the unaccompanied child(ren)/minor(s) until the unaccompanied child(ren)/minor(s) has boarded and the plane is airborne, and who shall confirm that the unaccompanied child(ren)/minor(s) will be met by another parent, legal guardian, or responsible adult upon deplaning at the final destination.
8) The parent, legal guardian, or responsible adult receiving the unaccompanied child(ren)/minor(s) upon deplaning at the final destination may be required to complete and sign documentation relating to such unaccompanied child(ren)/minor(s).
9) When two or more unaccompanied minors are traveling together, the most restrictive age requirement will apply.
10) Proof of age may be required by UA.
C) Unaccompanied Minor Service Fee
1) Service fees for Unaccompanied Minor service is subject to change at UAs discretion. The fare for Unaccompanied Minor service for children 5-17 includes the applicable adult fare in addition to a service charge of 99 USD/99 CAD assessed for each one-way journey from the childs boarding point to the childs final destination. Visit UAs website,
www.united.com, for more information about UAs Unaccompanied Minor policies, as well as tips concerning children traveling alone.
2) For purposes of this Rule, Unaccompanied Minor service includes reasonable supervision for Unaccompanied Minors from boarding until deplaning at the final destination.
http://www.united.com/web/format/pdf/Contract_of_Carriage.pdf
So, United requires someone under the age of 12 to be accompanied by an adult (18+ years old). As such, it is highly unlikely their online booking engine would permit ticketing a minor on their own PNR, and probably requires a call to the ticketing agents to book the flight, as well.
As a data point, this weekend, we booked our flights to Anaheim. I book DW and DD at separate times using Skymiles, and the system would not let me book our daughter online. I had to call a ticketing agent to complete the transaction and get the PNRs linked, thus avoiding the unaccompanied minor fee. So, I guess I do have a little bit of experience with this, but I knew what was happening due to reading the CoC.
we fixed it quickly so they were traveling with at least one of us.

It's much ado about nothing really... We've had issues with delays and changes and it's always been worked out ... so again, maybe if you don't know any other way, it seems ok, but not for someone who knows there is a better way
...The solutions are simple, it's people that get in the way. It does not HAVE to be this way, but it is the way they chose to do business. For me not good. I understand what is good for some is not for others and there may be benefits to someone else, but that does not change that this process is CRAZY!!!!

:
It isn't people getting in the way, it is actually the automation. Computers are unable to think and make the sort of intricate judgement calls you require for your travel. As you stated in the first paragraph, people were able to override the system, link the PNRs, and also make changes for irregular operations (irrops) to ensure your family is together.
As for the perceived complication of the system, yes, it
does need to be this way. The airline's reservation system is geared to make booking adjustments automatically and in real time to ensure people are able to reach their destination on day of travel. To that end, the system uses the PNRs to recognize and accomplish this action. What you consider to be a hindrance is actually safeguards to ensure the safety and security of minors during travel.
The legacy airlines have sophisticated frequent flier programs, as well as a much larger network than low-cost domestic airlines (like WN, B6 (JetBlue), and Spirit). They need to make flight changes due to weather in Japan and aircraft breakdowns in Dallas. It works very well, and to question the validity of it without respecting the intricacies of the requirements is a bit foolhardy and naive.
People do make it possible, but maybe your request is a significantly more complicated than you are aware and/or others have taken a course of action that may have been less safe for your family. In the end, I encourage you to fly with an airline that meets your expectations. If that is WN, then please stick with them. You vote with your dollars, and if you decide to pay more for an airline you are comfortable with, then that says a lot to the others.
To summarize...
The system works, and works well, given the shear number of changes that occur daily to flight schedules. Airlines have rules to protect themselves and their customers from actions, which likely occurred many times previously.
Just because an action appears to be simple and straightforward within a micro-view, there are other actions that could and would cause great pain allowed to move forward. The online booking engine has certain fail-safe actions built in to prevent the traveling public from harming themselves.
It is paramount for you to read the Contract of Carriage and Terms & Conditions before ranting on a public thread. The answers are available, easy to access, and provide a clear understanding of what the rules are. If you choose not to review the rules, then that is your prerogative, but then there is no basis for complaint.
Finally, please don't passively insult my abilities just because they disagree with your statements and point of you. If your only argument is to belittle me with no idea what my knowledge or experience is, then you may end up looking more foolish.
I wish you luck in your endeavors to get your family to WDW!
