do i have to be 18 to cross the border?
This answer assumes that you are an American citizen who is asking whether you can travel internationally. Although there is no specific law that requires a person to be of certain age when crossing the border, you should never cross the border without telling someone, like your parents, first. Additionally, many countries require that a juvenile has a notarized letter of parental consent if he or she is travelling alone.
Also, since January 31, 2008, the United States has required that U.S. citizens ages 19 and older must present documents that prove identity and citizenship in order to get back into the United States. The ID and documents must include your photo, name and date of birth. Children 18 and under, are only required to present proof of citizenship. To find out requirements on international travel, you can go to the Department of Homeland Security Website.