I'm 45. I regularly make friends with people who I meet climbing, at meet ups, at work, at social functions etc. Some of these have proven to be long term friendships. I don't think it's the "adult" factor, I think it's a choice of priorities. Probably the most important factor in my circumstance is my partner (of 26 years) and I don't have kids. Having said that, some of my more recent friends (last few years) do have kids, so they're obviously still prioritising meeting new people and doing new things.