When they start asking for it. My son started asking for "pie-see" when he was on the toilet at about 2 1/2 years old. He was about three when he first started asking for it while in the bath, so I'd sit right around the corner outside the door so I could still hear if he slipped or had any other issues. Now he's 6 and has been taking baths entirely on his own for about a year, including drawing his own water. My daughter is not yet to the age where she can be out of sight in the bath, so no idea there ;). What I have gathered from my own experience and from watching other parents is that it is a very individual thing and depends on each child's preferences, as well as their ability to do things for themselves.