I am 50, so I fit the demographic you are asking...
I like to run. That is the main thing. I think I would still run even if I were alone on a desert island.
I like my training partners, so running with them is also a two-birds-one-stone kind of thing. I get to be with friends and socialize while getting my workout in.
I like being outside and have pretty nice places to run, so it ticks the "get some fresh air, get into the woods" box as well.
I would be a fat pig if I didn't exercise. I like to eat and running helps me stay fit.
I like to race and take on other athletic challenges. I want to be able to run a 5:xx at 50 and a decent marathon (BQ?) at 60, and stuff like that. I don't really race for PRs since I am a former D3 runner and even though my PRs are slow, they are untouchable now. Nonetheless, I like to race and do all-out time trials with my friends (which feel like race level efforts).
Running helps me to be less stressed in life and makes me happy. I am a better dad and husband because of my running.
Those are the seven reasons I run.