I went to
Disneyland with my son just after he turned 1. My family is from So Cal, so we were visiting the area and went for one day, not a vacation just to go there. It was fun because I love Disneyland...honestly, though, it meant nothing to him...I mean sure he thought some stuff was fun, but he thought the local Farmer's Market was fun too
We have not been on any vacations since...
We are going to go for my son's 3rd b-day. We will get there when he is 2, so still free on all accounts (AND we can have 5 in a room).
Some people say older when they will have memories. Studies of memory will tell you that most people's first memory is somewhere around age 3. So maybe DS won't remember it, but how awesome would it be if when he is 70 Disneyworld is one the first things he can remember from his childhood?
Anyhow, if YOU like Disney and have the money, it is an easy place to go with a baby/toddler...if you want a magical "first trip" thing, I would say wait until your child has some language, so 2-3. (It is also theorized that memories begin to form more permanently at this age due to language development).
At nearly 3, my son is very verbal with a good memory...he is even looking forward to the trip and can tell you when we are going and highlights of the plans...this is much better than if we had gone a few years ago...and he is still FREE! (At 3, he also has enough life experience to know the characters and yet still really believe that we are eating with Mickey Mouse...at 18 months, this would have been a giant rat at our dinner table...at 8 this will be a guy in a Mickey Costume). So while I say just before 3, I think for something truly magical---between the ages of 2 and 6 for a first trip.
Sorry this is long, but also wanted to add that before the 12 month mark, he was easy to take anywhere (he would just nap however, still nursing, etc.)...after around 14 months until 2 1/2 there was a lot of training on how to act in public (hold hands, danger, waiting in line, restaurant etiquette, etc.)...Anyhow, it was fine to take him at 12 months (in a carrier), he will be fine at 3 years...it might have been iffy at 18-30 months in terms of naps/eating out/scared of things/can't go on certain rides, etc. So I have to disagree with the potty trained comment...Not potty trained/breastfeeding=easy to take ANYwhere....starting to potty train and walk and all solid food and cranky=difficult to do anything (a trip to Wal-Mart takes planning and skill
)...potty-trained, walking well for periods of time, enough language to begin to reason with (i.e. how to wait in line and why we wait in line, etc.)=easier to travel with again