I'm so glad you asked. I just turned 80, and even though I don't have the stamina or strength I had when I was 40, I've kept up my mobility through (more or less) regular exercise. As the old joke goes, if I'd known I was going to live this long, I would have taken better care of myself. My only saving grace has been (semi) regular exercise and a (semi) good diet.
And yes, since you asked, there definitely are some advantages to aging. That's why it's a good idea to stay in shape to enjoy them.
It's a continuum. Two years ago (at age 55) I made an effort to get into decent cycling shape and certainly was the fittest I'd been in a decade. But comparing to similar level of fitness 20 years earlier, I was still 10-20% slower in every respect. I say "was" because I've backslid since and aged another couple of years. I'd say you can maintain peak physical ability until about age 45 and from there it's downhill.
There are outliers, like the 75 year old who passes you on a hill climb. But he's probably much slower than he was 30 years earlier too.
Another limiting factor is you get more "injury time". A knee injury that the doctor said would get better in 6-8 weeks "if you were twenty years old" mostly got better over the course of a year. But that's a big chunk of time.
All this being said, if you enjoy doing a thing, you'll still enjoy it if you have to go slower or not as long. Just do it.
I'm nearing retirement age, but I haven't become less physically capable. I think I've become more physically capable than when I was younger, actually, because I'm more physically active now than I was in my younger days (I've never been entirely sedentary, but developed hobbies in my 40s that involved greater physical activity).
It depends on what you mean by less physically capable. I'm in my early 50s and can do several things -- like run, cycle, or climb -- better than when I was in my 20s or 30s. But I'm not as flexible as I once was and I do have a bit of arthritis in my fingers.
It varies from person to person. People love to take credit for their state of health, but mostly it is down to genes and luck. Some life habits will make things worse, you know what they are. But some people are dead instead of old, and you won't get replies from them.
> People love to take credit for their state of health, but mostly it is down to genes and luck.
There's plenty of evidence that combining a good diet with regular exercise can improve how long you may survive.
I'll turn 70 next year and am about to do my first single arm pull up. I think, if you set yourself a goal - even an ambitious one - and keep pursuing it, by defintion you will improve your physique and ability. Just keep at it.
