Organized religion, though it is on the decline, is also a very important vector for socializing. So are local sports leagues. The key is frequent exposure to the same people breeding familiarity. I'm in your boat, a little older, and married with kids. Having kids helps, because you take them to the same activities on a regular schedule which puts me in regular contact with other similar adults. They're acquaintances, not really "friends", but it's better than nothing.