I don't think this is a wholly abnormal thing. I also have a friends group that has lasted since my teens. Though, as they become more reactionary over the decades, we see each other less. Adolescence is probably the freest time in our lives, or at least it should be.

I think the thing about friendships after your 20s is that instead of meeting people at school, you're doing it at work. If someone turned out to be shitty in school, that was far less consequential then if someone turns out to be shitty at work.

You need to work to live, and compromising your work experience by getting too interpersonal is a real concern. In school there is a lot more comradery between students. At some level, none of you want to be at school, and none of your peers hold any authority over you. Most everyone in your town goes to your school, and you are never worried about getting expelled. There is very little risk in building social bonds in this setting comparatively.

You would think work would be the same, as you all likely do not want to be there on some level. However, you need food, and shelter, and healthcare. At jobs that hardly provide those things, I imagine its easier to make friends, since you're already getting so little to begin with, your always on the hunt for new work anyway.

As I've gained more security through my carrier I've absolutely become more likely to keep my coworkers as mostly coworkers. I've also been burned a couple of times over the years, which probably informs my position more.

As children, you have so fewer hangups and zero social awareness. As an anecdote, we went trick or treating yesterday, and at one house was a little girl and her grandma. My wife got to talking with the grandma and my kiddo and her grandkid started goofing around, playing with our youngest in his stroller. The girl was asking us questions about our costumes, and telling us about her night getting candy. By the time we left, this girl, who none of us knew 5 minutes ago, gave nearly everyone in our group a hug.

I would have to be intoxicated to be as social with strangers as this kid was. It was very cute, but I think, very telling about kids and how they form bonds. Kids are naturals at bonding. Over time that instinct gets clouded by our own lived experience. We become so bogged down by our own fears and worries that we become guarded around others. So much of our socialization is bound to our labor and how we labor. Bonding at work is warped by the social relations of wage labor. You have to be able to work with people at the end of the day. If there is conflict between you and someone else it could cost either of you your jobs. You won't move up a grade next year and you won't have a 2 month break in the summer to detox from the incompatible people you deal with on the regular. There are a lot of consequences to be had socializing at work.

We have very few consequence free places in which we can exist and be our true selves. Where we can socialize like we did when we were in grade school. Its magical watching my kids socialize. They make it look so easy. Its because for them, it really is.