I exercise a lot in my 40s and have always done something through adulthood. My knees were in better condition after my marathon than they were as a teenager after DoE expeditions. I'm fitter than I was when I was half my age.
I lost my dad in his early 50s to heart disease that was aggravated by being sedentary, stressed and overweight. Giving up smoking after a heart attack at 48 wasn't enough to undo the damage.
DM is in her mid-80s now. Her life has been a bit more active with housework and childraising, but she's spent the past 20 years being restricted by osteoarthritis. She broke her arm slipping on ice in her late 50s, and her leg slipping on a wet path in her 60s. Her mum (who was born 110 years ago) didn't do anything beyond essential movement in a generation where life was still manual. She lived to 90, but from my memories in her 70s onwards, she always had that frail, stooped old lady look and was pretty much housebound for her last 15 years or so.
MiL in contrast managed to remain functionally active until her late 80s. An underlying condition caused her final decline, but it was only the last few years when frailty set in. In her 70s and early 80s she busied around and commented on "old people" that were younger than her who had generally given in to an old mentality.
Exercise (and diet) is no magic solution and I don't want to live forever anyway, but having a maternal line that lives longer than average (a lot of relatives have made it into their 90s), I want to live well through my life and not spend the last quarter wasting away.
I enjoy outdoor and social exercise and that has a huge amount of mental benefits as well as the physical. Even if some random curveball ailment gets me, I'm living well and enjoying what my body does for me. Being in my 40s, I'm seeing the gap open up between friends that have healthier lifestyles and those who don't. DH thought he was doing fine then got a shock when doing an outdoor activity and finding it much harder than he expected. He's improved his regularity at the gym, and while not being "A Runner" has started going to parkrun regularly and is really noticing the benefits and feeling more energetic generally.
I don't know what the future holds, but maintaining my bone mass, building muscle mass and keeping a strong cardiovascular system is the best way to enjoy life and delay the onset of poor health as much as is viable.