I think you're severely underestimating what 20 years mean for your body. I'm a very fit 37 year old, and I very much notice the continued wear and tear. Recovery takes a lot longer, and the body just isn't as limber as it used to be. Partying for days on end, or some extreme sports, and maybe even some intellectual persuits are just WAAAAAY harder then they were 20 or 15 years ago. I can still do them all, but someone who thinks it's a good idea to trade in their 20s for more free time in their 40s is in for a rough surprise.