I walked into the living room earlier to see my 6 year old playing with his willy, when I say "playing" it seemed quite grown up what he was doing so remembering discussions on here I said to him "can you go into your bedroom if you want to do that please" and he grinned and said "ok" (and stopped doing it).
I came away thinking maybe I shouldn't have said anything since he's only 6, what do you think? start as you mean to go on?