To secure a conviction you would have to prove the man in question was aware of the age of the girl and that she hadn’t lied or deceived him (the easiest defence to make) - this would apply in cases where statutory rape could not be secured. What you could do, and which should be done - is make it the responsibility of the man to get proof she isn’t underage - however, he could simply say she produced documents - and you’d be back into reasonable doubt territory. One would hope even the thought of a second trial might dissuade someone from committing the same crime twice - but many paedophiles act on compulsion regardless.