At what age can a child be in the bathtub alone?
Or is it a skill set they need rather than an age?
How about for a short period of time, like to go get something from another room?
How old do you like your child to be, or at what stage of skills do you like your child to be before you leave them in the bath alone for a minute? How about for several minutes? How about to do the majority of the bath alone?