I would argue that schools are responsible for teaching sex education. In an ideal situation, this education would take part in addition to the efforts of the parents in informing their children about sex. However, many parents may not feel comfortable in doing so or may not have the extensive knowledge necessary in detailing the causes and effects of specific STDs, HIV, how to use contraceptives, etc.
Sex education should be an ongoing process but should begin no late than fifth grade (age 10). The taboo on sex in North America is absurd. Humans should be educated not only about the physical act of sex but also gender and sexual identities. By this I am referring to the necessity to remove taboos on sex by informing children about masturbation, heterosexual couples, same sex couples, and people who are transgendered of transsexual.
I wholeheartedly believe that much of the hate, anger, frustration, and sexual repression existing within North America could be improved through educating children about sex and sexual identities in a complex and inclusive manner.
The one aspect of sex education I would exclude, however, is abstinence-only education because this is misinformation and tells human beings to repress and deny their natural sexual desires.