I make friends as a an adult like I did as a kid. Doing things I like to do, and finding like minded people who do the same. This means that as my activities change (i.e. I try a new sport for 5 years then quit), then I expect most of the friends I made through that activity will probably move on. That's cool. That's life. Roll with it.

For example I started lifting again after a break of many years. I now have a range of new friends from people I just wave to, people I chat with, and a somewhat regular training partner. Very nice.