End Game

I was thinking today about the what I hope to accomplish from the Mudder-A-Month challenge and from the Tough Mudder in general.  There's no great insight here - mostly I just want to be helpful and raise some money, but there's the ancillary benefit of being motivated to stay fit.  This got me thinking about some of the people that I've seen running Tough Mudders - and not just people my age or younger, but the people who are quite a bit older.  I was wondering if I would be running Mudders when I'm in my 60's.  There's absolutely nothing that would keep me from doing that, I think.  But, it got me thinking about the nature of the person who is doing this kind of thing when they're getting to that age.  Do the start when they're in their late 30's like I did, or is this a lifetime choice of testing themselves from the earliest days?  I like to think that I was an active kid, but the reality is that I was much more bookish than I was athletic.  That's not to say that I wasn't active.  I played sports in High School, and I remember being outside more during the summer than being inside, but the desire to test myself physically seems like a relatively new characteristic. 

I wonder if it will fade as if it were a passing phase, or is this a real lifestyle change?  I guess I'll really only know if I'm 65 and running a Mudder.