I saw a grown woman pick her nose and eat it earlier. 
Okay, so I never understood why people ate it. What is it about it that makes them want to do that? I just flicked mine. And I thought it stopped at a child's age. Really interesting.
I was so grossed out by it.  I think it has to be a oral fixation or something like that.
I've heard of oral fixations before, but only in the context of like chewing fingernails and smoking cigarettes, etc. But I guess if I think about it, a lot of people chew fingernails, and I'm sure that's just as bad bacteria-wise. Maybe the booger eaters are on to something.
Maybe it helps build their immune system.