Nobody need investigate if there is no complaining party. Live and let live. If the young girl later changes her mind and decides she's been manipulated or harmed in some way, then perhaps the law should get involved. Until that point, it's really no business of anyone's.

And it's fine if you don't wish to live in "Libertarian Land," no libertarian would force you to. You're welcome to surrender 50% of your earnings and let others impose absurd restrictions on you if that makes you happy. The problem is that the non-libertarians don't afford the same courtesy, and force the libertarians to surrender to their tyranny by "voting" for such.