I'm 41, and I just started Geocaching. However, this seems like something I would have enjoyed when I was younger, but it didn't exist! (And then I didn't really know about it for a while.)
From what I can tell from pictures, there's a good range of people geocaching in my area... but maybe slightly skewed away from my age in both directions, toward younger families with kids, and older folks. Older than me, anyway.
I think it's pretty great that this activity can be enjoyed by so many different people. It seems like each person is into it for a different set of reasons.