I was pretty much friends with everyone in high school, but didn't have many really close friends. I could pretty much sit down at any lunch table and be accepted, but I didn't make any lifelong friends in high school.
In college, I had a smaller circle of closer friends. I basically had two people that I hung around with all the time. I felt a lot closer to them than I had been to any of my high school friends.
I think that I really learned what friendship is supposed to be about when I got to grad school. While I had felt really close to my undergrad friends, I realized pretty quickly after graduating that those relationships had been more superficial than I'd thought. I tried to maintain those friendships, but it was pretty much "out of sight, out of mind" from their side. Through my first year of grad school, I realized that neither of my undergrad friends were interested in working toward any kind of continued friendship and I eventually gave up on my own efforts. This was honestly a pretty tough thing for me to go through. For years, I'd thought of these guys as brothers and I just kind of assumed that we'd be friends well beyond our college years.
In grad school, I finally found a circle of friends who really cared about each other. It was the first time in my life that I had friends who would go out of their way to do something nice for me without expecting anything in return other than my friendship. If I happened to mention during lunch that I was moving into a new apartment on Saturday, a group of classmates would show up at my place out of the blue on Saturday morning with a borrowed pick-up truck and they'd spend the whole morning helping me move. No need to beg, or even ask, for help. Just people doing nice things for each other out of friendship.
So I guess generally speaking, my relationships have progressively become less superficial through the various stages of my life.