Maybe it's just me, but I tend to identify my friends by where I know them from. One named group that I think many people have is "college friends".
I find that every time I hang out with my college friends, there's a sense of home. I left my college town the day after graduation and haven't lived there since. Many of my college friends get to see each other every weekend, but I don't get to reminisce very often.
I saw three of those friends this weekend and it's a pretty cool feeling to spend time with people who knew you "back when (fill in the blank)".
People change and grow up and all that, but friends are a bookmark in your life that help you flip back and remember "back when".