Libertarianism looks attractive in theory. But the flaw in libertarianism is that it doesn't account for the fact that adults' decisions and behavior impact the children who live in their homes, neighbours' and the greater community.; which in turn affects everyone else. 
Libertarianism also makes the false assumption that adults who are destroying themselves with hard drugs are "consenting". A heroin junkie isn't really consenting, they're addicted. They can't help themselves, they destroy their entire lives looking for their next fix.