I still don't leave Harry alone BUT Its not about age, its about maturity and water sense.

Harry jumps around in the bath and slips a lot. He needs me there to catch him so he doesnt hit his head and end up under the water. If he didnt partake in risky behaviour then he would probably be fine for brief moments.

ETA: It might also have something to do with Sams near drowning. I sit outside the door when he is bathing now too.