My eldest is 2 and is definitely not left alone in the bath for more than a couple of seconds. If I have to leave because I have forgotten a towel, I get him to talk to me until I get back in the room. That way I know he is OK. Luckily he loves to talk.

I would probaby allow him to bath alone when he is about 6 or so but I think I would make sure I am just outside the door. It will be interesting!