I stayed in US for two years with my B-2visa but came back to my country before i turned 18.

When you entered the U.S. with your B-2 visa, you were issued an I-94 form which should have been stapled into your passport.  The I-94 form has an expiration date which means you can stay in the U.S. legally until the expiration date.  It is possible to extend your stay by filing an application with USCIS before your I-94 expired.  If you did not file an application to extend your stay, you fell out of status when your I-94 expired and your B-2 visa stamp was automatically revoked.  In this case, you would not be able to use the B-2 visa to enter the U.S. If you were granted extensions of stay for the whole two years that you were present in the U.S., you can return to the U.S. with the valid B-2 stamp in your passport.  You should present copies of the approval notices for the requests of extension of stay when you enter the U.S.   Good luck, Mark