I recently read a story about a 19 years old boy sentenced to jail and to be registered as a sex offender for 25 years for having sex with a 14 year old girl who lied about her age. The girl and her mother testified on behalf of the accused, saying that the sex was consensual and that she told him she was 17. Unfortunately, that didn't matter.
I read that if she showed him a fake ID so well done it's impossible to discern from a real ID, that wouldn't have mattered either. I also read that even with notarised written statements you will still be found guilty. (Curiously enough the reverse, ie propositioning sex to a person who is over the age of consent but lies saying they're younger, is also illegal)
Radiocarbon dating for samples less than 100 years old is so unreliable it's basically useless. Even if the person looks very old, there are certain diseases that cause rapid ageing so they potentially are underage.
With all that in mind, is there any way to have sex on US soil without running the risk of being labeled a sex offender?