Well, it doesn't appear to be a violation of the Mann Act of
1910.
http://www.usdoj.gov/usao/eousa/foia_reading_room/usam/title9/crm02027.htm
...transporting any individual, ...for the purpose of
prostitution or sexual activity ...transportation of minors under
the age of 18 years ... provides for an enhanced penalty.
See also emancipation
http://www.steveshorr.com/child.support.military.service.htm#Emancipated%20minor;%20description
heaps of people have done it so i dont think it would be a
problem good luck