My son was about 7 or 8 when he started bathing solo. I always made sure the water was not too hot, and only a little bubble bath was used. And until he was this old, a lot of times he didn't think of washing the dirt off much. A lot of times as he got older I'd sit near by and read to him while he washed himself. Why so old? Well, let me put it this way, when I was 5, I got a hold of my Moms safety razor and shaved off my sisters eyebrows. Because a younger child might not have to sense to wash thoroughly, not flood the bathroom, or fill the room up with bubble bath suds. If a child slips, falls, hits his head, they could drown, even at five, and waiting till you heard silence is NOT fast enough to respond to it and soapy water in the lungs is NOT good at all. A little prevention is worth a ton of mess (or water damage) or injury. The most dangerous rooms of the house are kitchen and bathroom. I'd much rather err on the side of safety.