My experience has been that as I aged, I became injured more easily, I got out of shape faster when I was injured, and it took longer to get back into shape as the injury was neutralized. There are three other factors that can complicate this.
1. Physical. Our bodies remember old wounds. You might say they hold grudges. As well, cartilage breaks down and we lose bone density and muscle mass.
2. Situational. Our mid-life/end-career flow means we are often denied the benefits of regular unit PT when we need it most. The added time crunch of families (the ones the Army didn't issue us) hits at the same time, just as an added bonus.
3. Moral. "The moral is to the physical as three to one" - Herbert W. Kornfeld (just kidding). My knees know I'm fifty. My neck knows I had a tree broken over my head while driving a Lynx. My spine knows I was run into and tossed 15 feet by an M548 (not it's fault, an M1A1 hit it). My arches know a Leopard C1 tank track was dropped across my feet.
But my brain does not know these things. My brain keeps trying to tell me to do things without the prior training, preparation and stretching my body needs to stay serviceable. My brain thinks it is living in a 19 year old body on Basic Para at CABC Edmonton. My brain wants my body to wake up drunk in Germany on a Sunday morning and drive to France and run a marathon the same day, on a Pils inspired whim, without the benefit of training..
My brain can no longer understand why my body cannot do this.
After all, it's done it before.
So, to make a short story long: Try and do the bulk of your physical sojerin' before you are old enough to get on the TTC buses for free.
I was probably in my best shape between thirty and thirty five years old.
It is a wonderful life, full of great people, but if you do it right - pedal to the metal (driving your life like you stole it) - it takes a toll. Physically. At least, physically.
Ah, but the stories you will tell!
"Old age is a train wreck." - de Gaulle