I don't think it is legal anywhere; only the definition of "underage" may vary, between 16 and 18 years. Here you can see the breakdown by state:
http://en.wikipedia.org/wiki/Ages_of_consent_in_North_America#United_States
Note: The age of consent refers to an adult having sex with a minor - this can get the adult into real trouble. Sex between two minors is usually less problematic.
Underaged sex
Same-sex marriage is legal in the US Virgin Islands effective June 26, 2015.
Same-sex marriage is legal in all 50 US states.
Yes, beginning in June 2013, same-sex marriages are legal for the purposes of US Immigration Law, as long as they were legal in the place where they were performed.
No. Civil unions are not legal in the US Virgin Islands.
No, it is illegal in all of the US and its territories.
You have to do it through your parents or legal guardians, since minors can't enter into contracts.
Yes. In fact, same-sex marriage is legal in all 50 US states as of 2015.
Same-sex marriage is legal in all US states effective June 26, 2015.
When she reaches the age of majority for her state, age 18-21.
Yes, same-sex marriage is legal in Oklahoma, in all US states and territories, and in some foreign countries.
it is legal for you two to date and in most places to have sex.