In support of "notmyrealnick" I have to say that most people wrongly believe that the sexual life of humans only starts when they reach adolescenthood, Bronislaw Malinowski in his studies with savages(Book: The Sexual Life of Savages in North-Western Melanesia) showed that the starting age can be as young as 5 years old. But we in our modern society repress the children.

http://en.wikipedia.org/wiki/Bronisław_Malinowski

Edit: related to this is the (IMHO wrong) thought that underage humans cannot possibly give informed consent to sexual acts.

Edit2: Btw, when I speak about underage sex I'm thinking about sex where all the involved are more or less of the same age.