Unfortunately, they get the mindset that they're "too old" for Disney, and it's very hard to shake that feeling. I had this issue with my last trip. It was my girlfriend and her 3 teenage children. Strangely enough, the oldest was the one who looked forward to the trip the most. The other two were rather "meh" about the trip.
What I had done was to try to get them involved in the planning, and tailor the pre-planning aspects to what they already like. One of them really likes Italian food, so I made a couple of Italian restaurant ADRs. The other one can subsist on McDonald's, so I kept a couple meal times open for a random QSR that the kids could pick.
The FP+ was a mix of what they wanted to do and what are the "must do's." Thankfully, the oldest's choices of rides were all "must do's," so those are easy. My Italian one wanted to stay on Terra Firma a bit more, so I picked a couple easy rides and also went on some of the slower rides. McD wanted more roller coasters, so in comes 7DMT, Expedition Everest, and so on.
The key is, get them involved in the planning. Even if they only give nothing more than vague answers, try to make them into something concrete. It'll make their trip better knowing that something that they want to do is on the horizon, and that they had some say in planning the trip.