No, you will not "automatically become US citizen at 18". If at least one of your parents became a US citizen by naturalization, and if you live with that parent, it is likely that you are already a U.S. citizen and only have to apply for a certificate of citizenship or for a US passport. If, at least, one of your parents is a US citizen, call or e-mail me, and we can determine whether you are already a citizen by operation of law. If you don't have a US citizen parent, you can apply for naturalization as soon as you have your 18th birthday. USCIS will make you take a test and go through an interview and an oath ceremony; but it will not happen unless you file an application (Form N-400 - you can find it at uscis.gov).
Answered on Jan 14th, 2013 at 6:55 PM