Sexual maturity has nothing to do with being the least bit culpible in this. Just because he has the physical maturity to get an erection doesn't mean he bore any responsibility whatsoever.
Would you say that a 11 or 12 year old girl would be responsible for seducing an adult man just because she had entered puberty? I think not. Teenagers are suceptible to being manipulated by older more experienced individuals... man or woman doesn't matter. That's why there are laws about statutory rape on the books.
And statutory rape happens far more often than gets reported.