I've always been called mature and older than I seem even when I was very young. Among my last friend group, we were always cracking jokes about how I must be a million years old or how I'm a boomer (I'm 29.)
It tends to be related more to experience than literal age, I find. I've done and seen a lot of things, held a lot of jobs, and have a lot of skills. I'm not a master of everything but there's only a few things I can't do adequately. I have no metric with which to gauge how much experience I have, but certainly I do feel as if I've experienced more things than most other people have, broadly speaking. So I guess I do have an old soul, like I've crammed 50 years of experience into a 29-year old body.