When I first moved to Maine at 21, I met some folks through bars and the like, but most of my leading friendships from that time came from my main hobby. I joined a couple of running clubs, and most of the people were at least a decade older than me at the time, but I started to meet their kids and acquaintances, and the clubs I was in at that time have plenty of younger folk now than they did then pretty much across the board.

For organized clubs, the Maine Track Club is pretty large and had folks all over the state. Trail Monster Running is smaller but is very inclusive to anyone wanting to join us even if they never become a member. I know there is a pub run group in Portland that meets regularly but they didn't exist when I lived down there so don't have a lot of details. In the summer, there's a weekly 5k every Wednesday night around the Back Cove that provides a good opportunity to socialize.

I imagine there are similar opportunities for other hobbies than running with young folk. Some of the book clubs I attend have younger folk, though most of the town governance or other volunteer stuff I do lately skews towards an older demographic.