Seems to me that the sex desire is caused by a virus.

When you have a child, say, preteen, the child can get a lot of diseases that adults don't get so easily. Take measles, for example. An unvaccinated child can get measles fairly easily, while an unvaccinated adult who never had measles doesn't usually get the disease. This is because the immune system of the adult is more fully developed, and can fight off diseases better.

By the time people turn 87, their immune systems are so fully developed that they can usually fight off even the sex virus. (chuckle)