The arrogance of youth and the price we pay for it later.
People say stretching is enough, but Lord knows its not. The body just simply breaks down over time. So, if you want to continue doing the "fun" stuff, you just do it differently.
Last night, I had a very hard sparring session with men half my age. I am the oldest guy in that class at 45, and ain't nobody my age trying to stand with these young killers. But, I have years of experience, so I just do it differently, and love tee-ing off on these young guys.
Weight training - the heaviest weight I pick up is deadlift weight and bench press weight and that's 100lbs. That's it. But, the true beneficial weight lifting is lighter stuff, in the form of kettlebells, steel mace, sand bags, resistance bands, and body weight. The heaviest of those things is 35lbs.
Back in my youth, I could dumbbell shoulder press two 100lbs in each hand. There is no waaaaayyyy, I could do that now, nor would I want to.
I've come to grips with adaptability. I know this train is headed for a dead end, so I will do what I can to enjoy what I can while it's good, albeit painful at times.