It depends.
I am 20 and I have always been mature for my age! Growing up, when I was 5 people thought I was 10. When I was 15 people thought I was at least 18, etc. I got engaged at 16 and married at 18, which many people consider "young" But I've always been very mature, I've always known what I've wanted, etc. Plus the way I deal with and handle things are that of someone in their 30s/40s.
On the other hand, I have a "younger" way of talking. I've never mastered the "sounding mature and elegant" thing.

I love cartoons, I like to stay up late, I like cute clothes and funky designs, etc.
So on one hand, I'm very mature. On the other hand, I'm a big kid.
