We moved around a lot, so I didn’t really have a best friend until I was 13. Oddly, we met when I was hired by her mother to babysit she and her brothers (she was 11, and one of her brothers was 12, isn’t that weird?) I turned out to be the last babysitter they ever had, (they clearly didn’t really need one), and I didn’t see her again for three months because I had bronchitis all summer long. We ran into each other at the neighborhood playground, remembered each other, and hung out all that day. We were best friends by the end of the evening. We rarely see or even talk to each other these days, but we’d still drop everything to help one another, over 30 years later.
My other best friend is the one that @Fly refers to, above. We met when I was 21 and we’d both moved into a brand new neighborhood, right next door to each other. We were just nodding acquaintances for a long while, until I stayed late to help her clean up after a neighborhood party at her house. We talked all night long, and were amazed at how well we ‘clicked’. We are still best friends today, long after we moved on from that neighborhood. As @Fly said, our children are best friends, too.
Part of the problem with making friends when we’re older is that we don’t do the same kinds of things we did when we were young. We pretty much work and go home, and that doesn’t give a lot of opportunity to make new friends. Maybe if we put ourselves out there, out in the world a little more, it’d be easier. Go to meet ups and such, I don’t know.