I also have one older DD (now 16) and two younger children.
She actually still loves doing the rides and has always appreciated having the younger ones along as an "excuse" to enjoy all the "little kid" things that she would want to do anyway.
Because the younger two were toddlers on our first trip, we planned an evening where older DD and I went to pm EMH at MK while DH stayed back at the resort and got to bed early. It was really wonderful just spending some quality time alone together and letting her decide everything to do spontaneously. It's a nice break from the planning and compromise of making sure everyone in the group is happy. This evening out has become a tradition.
It's surprising, but she actually enjoys more things as she has gotten older rather than less. The kids were never really big on meeting characters, but last trip DD had to meet Merida. She picked out a special outfit and was the first in line. I have never seen her light up like that. She will also wear some of the fancy ears and even bought matching Star Wars shirts to wear with her little brother.
Not sure if this teen is already a Disney fan, but maybe check out "Disneybounding". My kids are already thinking about outfits to wear for our June trip.
Check out the Disney YES individual program!
I was going to suggest this as well.
Not sure if tickets have already been purchased, but it may be worth looking into if they haven't yet. (YES tickets include a 3 hour class plus tickets for however many days you will be in the parks at a discounted rate). You may also be able to find a class offered at the same time for the younger two.
My kids all did classes a few years ago and we have classes booked for our trip this summer. They are all taking Animation History at HS this time (separate sessions for teens and younger kids, but offered at the same time). It's really a great experience.