Generally speaking Libertarians do support an age of consent. The
Libertarian Party (www.lp.org) is weird though. The platform veers left and right all over the place. I consider myself a Libertarian of the
old school variety -- a strict adherence to all
Constitutional laws. Where laws are unconstitutional they needn't be followed. An age of consent has been found to be constitutional in several cases (though I couldn't name them); therefore, I'd think the age of consent laws on things like alcohol and movies wouldn't be contrary to Libertarian ideology.
Re: heroin -- it's easier for a kid to get heroin than it is for shem to get alcohol or cigs anyway. Do heroin dealers card people? That's why the drug of choice amongst teens is reefer -- easy access.