I did some more research and found that this subject is unreasonably complicated. There are many instances where a federal age of consent (18) is recognized, but there are weird exceptions as you mentioned: http://www.solresearch.org/report/US_Federal_Age_of_Sexual_Consent

However, kids are getting charged with prostitution who are as young as 10, so apparently our court system doesn't care about state or federal laws on the age of consent.