Around 7-8 years
There is no perfectly safe age to take a bath alone in a tub, but kids under 4 yrs are at higher risk of drowning
Statistics from a few years ago show that 70-80 kids drown in bathtubs in the U.S. Each year. To be safest, wait until a child has good behavior, some common sense, plus the ability to easily get in and out of a full bathtub before thinking of leaving him alone. Otherwise, watch him!