You don't know how much, when, how or why, but it will happen. I consider someone blessed if they make it to middle-age largely "intact."

When you're young and unencumbered and largely undamaged you need to use that time in any way you can; inevitably you'll end up "walking wounded" one way or another--less than you were--unable to return to who you were before whatever(s) happened.

When I was young I'd encounter older men that struck me as "defeated", tired, incurious, dismissive, and I'd never understood why until the last ~half decade or so of my life.

I really think human "life" and vitality is something that does get "spent", often against your will.