One factor that seems to be being overlooked is the difference between regulation on the federal level vs. the state level. The Constitution of the United States defines what the federal government can, and should, do, however each state has its own constitution stating the state governments powers. This means a person might be libertarian in their general philosophy, and still be a constitutionalist on the federal level. The Constitution defines boundaries on government, not requirements, a consitutionalists doesn't want to do everything simlply because it's allowed, he want to stay within the boundaries that it lays down.
First off this whole discussion is in regards to government interference, not necessarily voluntary systems like if a movie theater decides for themselves. In regards to an age of consent, probably most libertarians feel some age of consent is needed, the main argument is what should be covered. An age of consent exists because minors are considered unable to make fully educated decisions and so the question is what decisions are they too dumb to make. To look at this question from a pragmatic viewpoint: what decisions might minors make that will hurt them?
Movies - The main argument for restricting movies is that they contain violent or sexual themes that minors shouldn't see and that could create violent or sexual thoughts. Personally I don't think movies should be restricted because: 1) There doesn't seem to be a very strong case for a causal relationship between watching violent movies and doing violent things. 2) It doesn't work. Kids are going to find ways to sneak into R-rated movies unless theres a total lockdown, especially with the new ability to just download the movie off the Internet.
Even if you do want to restrict, there is no basis in the constitution for a federal regulation on the matter. A federal rating system a la the one currently used by the MPAA could be required for any movies traded across state lines (almost all of them). But as to an age of consent for movie theaters there's no federal constitutional avenue.
Drugs - There are a number of arguments for the government restricting drug sales to minors. The most prevalent is that they are dangerous. Minors are, theoretically, not able to fully understand the consequences, risks, or dosages and could mess up their mind, body or somebody else's property. Despite being a minor myself, I do believe this is a valid reason for regulating drugs. It is true that minors will still have access to drugs, but there will be a barrier that has to be crossed to buy them. The amount of time it takes to get a fake ID can be used thinking about the consequences.
However from a constitutionalist perspective regulation must take place on the state level, unless there is an amendment passed.
Sex - The argument for regulating sexual behavior is also that it is dangerous. The danger in this case comes from either the possibility of an STD or of getting/getting someone else pregnant, both of which are very bad for a minor. There is also the possibility of psychological effects.
However you have to consider exactly what the age of consent would mean; normally it means the age at which you can be sold it - but this isn't an option with sex (excepting prostitutes). So the question comes down to the equivalent of statutory rape laws - jailing people for having sex between two consenting minors. I don't see any way this could have beneficial qualities, as most kids feel it's private enough not to get caught (they have to watch out for parents anyways). Basically you end up with some jail time and the ruining of a couple random kids lives for no practical gain to either the kids or society in general.
If you really wanted to enforce an age of consent the only thing sold for sex that it could apply to would be condoms; however prevention of people having sex from buying condoms is really something we don't want to do
A pragmatic outlook on individual issues, more than simply blind obedience to some manifesto is what i like about most libertarians more than those who call themselves conservative or liberal. Just trying to be a reasonable guy.
Also being a constitutionalist libertarian I am against any prohibition on abortion however I don't support Roe vs. Wade on both grounds. The decision is not only at odds with a strict interpretation of the Constitution but also increases the government's power, that of the Supreme Court by allowing them to see things in the constitution that aren't actually there