adults????

My life very much relates to this subject. i am the youngest in my family by 9 years. my brothers are 28 and my sister is 27 and i am 18. when i act a certain way my siblings usually tell me i am acting like a high school student which i am, but when i try to act a little older my parents tell me i can't do some things because i am to young. with them there is no happy medium. i am either to old to act one way and should be acting older than i am, but when i do i am to young to act that way.