In England, "mixed gender football" is permitted to 18 years old
Netherlands and Switzerland allow mixed gender football until 19, Germany and Italy at 17. Bear in mind this doesn't just apply to professional football, but to any organised matches at all under a league structure. So girls and guys can no doubt have a kick about in the park, but can't play together in any organised league at any level
So it seems possible that, in England at least, should a star female footballer be identified at an early enough age, she could in principle play for a professional men's team. Until she turns 18.