If they said you are too mature, they probably didnt mean it in a positive way. Its a good thing to be responsible, but you are only going to be 14 once so live it. Try to find a good ballance.
Mature is a good thing...it means you are very respective and polite in public...that doesn't mean you can't have your own since of style.....just be who u r and you'll do just fine!
Don't be stressed by it....
p.s Guy's like girls who are mature....more intelligent conversations and bantering...witty responses..trust me..I get along great with guys because I am who i am!