I still feel 18 though that was ten years ago... I read once somewhere that an 80 year old man, when asked, said he felt as if he were still 18... so I suppose age is relative.
When I was young, like 12-18 I always felt much older than I was... and the people I hung around with the most were anywhere from 40-98 years old.
My advice to you, and to myself if I could go back to your ages: Enjoy your youth while you have it, as you will never have it again. But use wisdom beyond your years in choosing your actions, as in the future you will reap the rewards of such foresight.