At what age is considered safe to leave a child unattended in the bath

[~clover~]

At 7 DD1 is now started turning on the taps herself. Our water is not hot enough to burn her, although I'm careful with DS & the hot water.
I have taught her to turn the cold on first. Always, & the hot off first. She is still supervised while doing that, but showers alone most of the time.
I also leave DS alone in the bath with her comfortably. I know her maturity, & I know if anything happened she'd there, if he wasn't ok, I'm never that far away.

My mum left DD2 - 4 & DS alone in there when we were moving so she could get some things done, but I'm not so comfortable with that. DD2 is too feral in the bath, I don't want her knocking him over. But I will go grab things with her in there. I only put about 3 inches of water in if DS is in there. When he gets out she can have some more.

I will not leave DS alone yet. I might grab a towel from next to the door or the phone. I won't answer the door, its too far away. I'll yell out to come in.

I do often put the 2 little ones in the shower to try & get things done. I feel a bit safer with the shower & sometimes I do just need that 10 minutes of peace. I'm always listening though. Always.

I will not be standing in there with my children til they are 10. I don't think that is very fair TBH. Jaz usually leaves the door open, or half open, but I remember how embarrassed I was when a baby sitter made me shower at 10 while the little girls (4&5) were in the bath & I got dressed & undressed in the shower so they wouldn't see me.