Far as I can tell so far, the difference between young and old is, how long you can get away with slacking off. Get old and you have to keep up the exercise all the time if you don't want your body to quit.
I would add being more injury prone, and for things to potentially go downhill fast due to an injury to that difference.
Speaking from experience with taking care of my late grandparents(both reached 90s) the worst thing I think for them, is for them to stop being active mentally or physically. If they don't use it, they lose it and lose it fast.
And this even applies to people significantly younger than that. I knew someone in his 60s, who tweaked a leg a bit in a sport and got a pinched nerve or psiatic nerve. Which then he babied it for a week or two or three, and ended up in excruciating pain which grew worse and worse untik he was almost completely bedridden for a while. It was bad enough he heavily considered going to the emergency room and had previously gone to a clinic for x-rays. And he feared he would never walk again. Meanwhile at the same time he had lost a significant amount of weight(something like 20 pounds in just a few weeks) and feared the situation could've been cancerous.
Where the cause was ultimately found to be the supporting muscles had atrophied away on the leg he had babied, which caused his main muscles to essentially lock up. It took a week or so to regain some mobility in the leg with physical therapy. Where his good/better leg, could push lift say 80+(100+?) pounds or so, and the bad leg like 10 or 20 pounds max. And months of therapy/exercise before it returned to similar levels of strength.
The old couple in this series are a great role model. They are keeping very active by their farming and hobbies and help with neighbors/friends. And they are keeping quite engaged in the community which helps their mental and emotional health. Not to mention how healthy their relationship with each other is.