Disney itself doesn't have an age policy that covers all
ECVs in the parks. They can't because there are plenty of kids out there who own/use ECVs in their regular lives (just ones that are their size/set-up for their needs). They will not *rent* to anyone under 18. Most of the offsite vendors also have the 18+ policy.
Randy's Mobility (contact info in the FAQ sticky on this board) will sometimes rent ECVs to older teens. Usually it's ones who use one at home and haven't brought it, but it might be worth calling them and asking if it's a possibility for your daughter. It will give her the most freedom at WDW and be the least physical strain on the rest of your party. Randy's always has an employee meet the customer with the
ECV (so even before WDW's changes in drop-off policies, they would not leave their products with bell services) which means they can evaluate whether a specific older teen can safely operate the ECV. The biggest drawback is that at WDW there are some queues where she'd have to transfer to a wheelchair to go through the queue (any queues that don't work for manual wheelchairs, probably wouldn't work for anyone on crutches either). At Universal there wouldn't even be much point in bringing the ECV as their official policy is to ban ECVs & power wheelchairs from all queues and from what I've been hearing, they've started enforcing this more. (Personally, I wouldn't go to Universal on this trip since it will be there in the future and there's less you can do there with any sort of mobility impairment, cast, etc.)
I've used a wheelchair when flying for a decade. There won't be any problem gate checking both mobility aids. The crutches will probably go in the overhead bin. The wheelchair is most likely going to be placed in the hold, however, on planes with closets above a certain size, you can ask for a folding wheelchair to be stored in the closet (I don't have the specific size in front of me). A personal wheelchair has priority over anything other than another personal wheelchair when it comes to items in the closet. The relevant legislation is the Air Carrier's Access Act.
Within the US I've had good luck with both Jet Blue and Southwest. I happen to like Southwest a little more, but Jet Blue generally is as good with wheelchair users. For me the no seat assignments is the difference. pre-boarding with Southwest usually gives me my choice of what seat will work best for me without having to explain to every airline agent that I don't want the seat *they* assume is correct for me as a wheelchair user who can't walk past the 2nd or 3rd row (the assumption is that I want a bulkhead aisle seat because it means not transfering down the row to the window; I prefer the window so that no one climbs over me and one row back so that I have the space below the seat in front of me since I'm usually solo). Regardless of airline do ask to pre-board. Talk to an agent at the desk by the gate to make sure you have all the tags on the wheelchair needed. They'll probably only let one other family member board with her (which you should take up if at all possible), but it's worth the extra time to pre-board and then to wait to be the last off the plane. It means she can get settled without being rushed and without feeling like she has an entire plane staring at her. Personally, I'd consider upgrading to Jet Blue's premium seating (at least for her and 1 other person in the family) so that you know she has some extra room. You can also request bulkhead, but that does mean you won't be able to put anything under a seat so won't be able to keep your purse or anything loose (this has included water bottles in my experience) during take-off and landing. Whether it's worth it for her is something she should think about and if she decides she wants bulkhead, make sure to tell them it's a medical need. They may tell you that it can't be guaranteed before you get to the airport and, if so, make sure to request it when you check-in. Airplane aisles are narrow enough that she probably won't be able to use the crutches to get to a seat farther back than the premium bulkhead (sometimes you can also get to that 2nd row in from the door). If she does end up in a seat too far back to access with the crutches/wheelchair, they have a special chair (known as an "aisle chair" or "straight-back") narrow enough to fit through the aisles that she can transfer to. It's a bigger hassle, but do-able. (If she would need the aisle chair for seats after a certain row, you may want to mention that if/when you requests specific seat assignments.)
I hope you have a wonderful trip!
**As a side note, my "you" has been more general to both you and your daughter. She can make any requests herself and do pretty much all my suggestions on her own, if she wants. Since she's 17, she can, and probably should, make the majority of the decisions about how to deal with her injury. Obviously she'd need permission to do something like try to rent an ECV from Randy's or if she wanted to move anyone else's seat, but there's no point in your doing any of it if she doesn't want to
