I agree with what seems to be the general consensus that has surfaced in this discussion, use your Passport. If you use your ID, I am not too sure how they will determine that you are actually underage.

Avoid the greyhound, Border Patrol checks that mode of transportation and a lot of people have been deported as a result of thinking that the greyhound is a safer option compared to flying, which is not 100% safe, but so far seems the safest mode of travel, and fastest, and sometimes cheapest.