You're still a young girl. I believe a person can have most of the physical benefits of a young person most of their life if they take care of their body, especially with healthy lifestyle choices. I plan on staying young into my 50s, 60s and hopefully 70s and even possibly 80s.
I'm actually 34, but I don't feel old. Age is just a number, if you think and feel you are old, then you maybe it is considered. I don't care I can be 65 or 80 and still feel like I'm in my 20s and 30s. I think feeling old is just in our minds and heats.
I've learned tell myself this and remind myself over and over again that:
No. I don't think even 60 is old these days, unless you want to be, whereas when I was young I thought people who were 50 were getting old! You may mean that you feel more grown up, mature than your contemporaries. That would be normal as we all grow up at different rates.
It's relative depending on who your talking to. On the grand scheme of things your just a kid. To me your still just a boy learning to find your feet. To a high school person you'd be old I reckon. Depends on who you ask. Life and perspective change with age. Don't hang onto one stage. Let it go and live the next. People only get depressed about being old when they walk into the future facing the past