When I used to want to bum my students out I'd tell them that their health after 40 depended largely on what they did with their bodies before they were 25. It's largely a use it or lose it proposition. People who keep on moving, working out, living a physical based life find that they still have most of that in their mid-50s, and people who don't, start to lose it in their 30s. Same with mental abilities, and social stuff. To the degree that you stop learning, stop trying, stop caring, you lose the ability to do so, to the degree that you stop trying new things, listening to new music, making new friends, and start to get stuck in a rut, then all you are doing from that point out is digging it deeper, and there does come a point where it does get hard - if not impossible - to get yourself out.