Just When?

This past summer I had an experience which I am still quite amazed by. In my apartment building there are some old storage rooms. Well one morning I happened to hear some sounds coming from there so, curious, I went to investigate. I was really stunned when I saw these two naked boys having anal sex. The thing was, I knew them from having seen then around the building, and I know that they could not be older then ten. But one was vigorously humping the other. I just couldn’t believe that boys that young could be doing that, and I still can’t. Just how old, or how young, can boys be to have sex?