Pubic hair distributes smell molecules by increasing surface area and is absent in children because they are not sexually active.

“So therefore, smell is important and then you come to the thing and I don't know... The American population is always, you can't talk about things too much but let's talk about pubic hair. That's not there for protection or worm or whatever, holy bolly, it's warm enough. And the children don't have it because they are not sexually active. So as soon as you become sexually active, you have to distribute your smell, your little molecules, as much as possible. So you need a big surface. So therefore, in the areas where a lot of these molecules are secreted, you've got a lot of hair to increase the surface and to select.”