I'm going to be 45 this summer, and I've been doing Kings Island since I was 2. I definitely cannot do the "ride til I drop" anymore, unless "til I drop" means halfway through the day. Keeping up with my kids, I've found my feet are killing me at the end of the day, the constant rattling from the coasters wears on me by the park's closing, and headaches are a bit more common. I can still do all the rides (I did age out of Vortex a year before they announced its closure) but now I don't get upset if I only get 2-3 rides on my favorites over the course of a 2-day trip. We're taking the kids to Disney/Universal in about two months, and I'm trying to exercise like crazy to keep up my stamina for the 30,000 steps a day I know we're going to have.
But it's actually been a bit of a blessing. With our kids, we have to slow down a bit anyway (although my son is 12 and does most the big coasters). And it's made me appreciate the park more. I love the roller coasters, but I also like stopping to get some pizza by the fountain on International Street, taking some time to enjoy Planet Snoopy with my 8-year-old daughter, take in a show and just enjoy the park itself. You miss a lot of beauty when you're just running from adrenaline high to adrenaline high.