One's sense of right and wrong is ingrained in children as they mature to adulthood. Family and societal values are generally a constant and are supposed to be a baseline for one to determine if behavior is appropriate and acceptable within a society.
If physical relationships with a minor is taboo where you originate from, it should remain taboo and in most cases feels wrong for your entire life regardless of what country you happen to be in at any given time.
No adult that has been raised properly and socialized in a society that criminalizes sex with minors should ever have to wonder if that activity is acceptable. It doesn't matter how mature the minor looks, behaves or how willing a partner the minor seems to be, based on one's ingrained sense of morality, the decision or course of action is clear. Changing/ignoring a moral or ethical position isn't something that is supposed to happen in adults for the sake of convenience or personal gratification regardless of the environment.
Some people it appears can justify anything in their own minds when they want to and are willing to defer considering how others will react should their deviations from the norm come to light. Being a member of a society or the group used to be key to one's survival and a cornerstone of mental contentment. Now-a-days, with the development of circumstances where the concept of the individual has superseded the importance of the group, it has become easier to be antisocial, despite the knowledge that the behavior is not condoned by the majority. "Play now, pay later."
Clearly Derfish's friend, should not be allowed anywhere near post pubescent girls if enlightened morality is applied.