Twenty or thirty years from now I won't be able to carry a 15k backpack as comfortably as I can today. That would take half the fun away from traveling. Maybe I can still trek and climb mountains and walk the length of a long beach, but I bet my knees would complain endlessly, and then I'd only be able to do one of those once a month; spending the rest of the days waiting for the pain to subside. I might still be able to stare at the stars and wonder at the moon, and solve puzzles in the clouds, but the years would dim my eyes and dull the colors I could perceive.
And so I do my kind of fun now. There would be time enough to be the kind of person the world wants me to be, and that would be when my body can no longer keep up with the kind of person I want to be.