There is a scientific reason for this. Over the past couple centuries the average age that a girl starts puberty has been steadily declining. It used to be that girls didn't have their first period until they were around age 16. Back then the average marrying age was younger than today so it wasn't uncommon for girls to get married shortly after reaching puberty. Men are programmed from countless generations to see the onset of puberty as the point where a girl becomes a woman.
The problem lies in the fact that the average age of menarche (the age of the first period) has dropped significantly to age 10-11. Couple this with the fact that modern culture considers people to be children not only through their teens but in some ways up to their early twenties, and you have a situation that simply didn't exist hundreds of years ago. Girls hit puberty, but are still considered children for another decade. The whole concept of being a teenager is actually relatively new. It used to be that you were a kid, then you hit puberty around the same time you were becoming emotionally mature, and BOOM, suddenly you're an adult. As I said before, countless generations of conditioning has trained the male brain to equate the onset of puberty with the point where sexual activity is no longer taboo. So it is kind of unrealistic to expect teenage guys to overcome a strong instinct that is buried deep in their brain.
Of course, this entire explanation is based on the assumption that your cousin is in fact, not prepubescent, and that she actually has had her first period and is beginning to show the first signs of womanhood. It is possible that she is a late bloomer and and the guy is just a creep, but I suspect that my explanation is closer to the truth than that. But even if that is the case, the guy is probably a creep anyway; it's one thing to have feelings that are hardwired into your instinct, it's entirely different to admit that you would act upon them when it is obviously a bad idea.