I'm 43 and already can't do some of the things I did in my 20s and 30s. I did the OU for years, but I don't think I could work full time and study at the same time now. I can go out during the week, but only for a couple of drinks and on the weekends I can go out either Friday night or Saturday night, but not both and I find most of my friends are the same. When we were all around 40 we all started going home around midnight so it's not just me who has a bit less energy than in my 30s
The only reason I don’t do any of this stuff now at 48 is because I have kids. Between school runs, working full time, trying to get to the gym, cooking, homework, swimming lessons, shopping, cleaning, running a house etc I can’t piss off to the library until midnight to write an essay like I did when I was single.
I don’t go out drinking because I can’t spend the following day in bed, only venturing out for a mcd’s with extra large milkshake. That and I don’t need to go out on the pull any more, so I have better things to do with my time 
It would be MN land claiming that they can do what a 20 year old can do at 47
I can still do cartwheel, handstands, back flips and somersaults. I regularly show up plenty of kids and young adults at the trampoline park 