I remember asking my MIL when do I stop letting my DS and DD bath together. She said, "You'll know." Shortly after, at bath time, I'm in the kitchen and I hear giggling. I go to the bathroom and I see DD flicking DS's penis with her finger and giggling. I knew then that was the right time. By the way, they were only about 3 and 4 yrs old. Another time, DS at bath time, says "My beans, where are my beans?" Apparently, his "beans" had shrunk up inside him.