As the parent of a daughter I have serious concerns about letting a thirteen year old girl buy and ingest serious medication without my knowledge or supervision. Seems like such a move undermines the authority of the parents.
That being said, it makes more sense, at least to me, to at least raise the minimum age to the age of consent in a particular state, otherwise a crime could easily be committed against a minor and I as the parent, as well as law enforcement, are none the wiser.
For example, an adult male could convince an impressionable underage girl to engage in sexual activity which results in pregnancy. All the pedo has to do is run to the store and pick up a pill and if the child remains silent, no one is the wiser. I understand its a pretty wild scenario but it could happen. And kids can buy all sorts of medications over the counter.