Can't remember for myself (sort of recall around 10 or so) but, I remember my son, between 4 and 5; I was reading a story to him, and I could see this rhythmic movement under the covers, then he said "Mom. Don't look under the covers." I asked "Why?" and he said "I don't know, but don't look under the covers." I didn't, but it was hard not to crack up. I guess all he knew then was that it felt good.