Well, there’s not much to go on. We don’t know you, we don’t have any access to your friends, and you didn’t tell any story that might help us guesstimate what’s going on.

That being said, a lot of the time someone gets pegged as the “kid” of the group, it comes down to either their relative age, or their behavior.

Some people are just naturally more childlike. Not childish, though there can be overlap. The kind of folks that are bubbly, or energetically happy, or tend to have a certain naivete, that kind of thing. It comes off as younger to some people, so they’ll start treating that person as younger than they are.

But the other part is that sometimes even a year of age difference changes things when it comes to perceptions and group dynamics. You’re 21, so if they’re 22-25, it really can be enough of a gap to set you up in their heads as the “young” one.

That’s the stuff that tends to be common enough to fit without knowing more, or assuming anything about motivations.