I normally hate "how old" questions, but here is mine
I have 2 DDs, 7 and nearly 6 and I still bath them together. DD1 has started to say she doesn't want a bath with DD2 anymore but it's just so much easier! DD1 has recently started to be much more independant, which is nice, but I suppose some part of me wants to keep her young a bit longer.
I think there is no set age as such, its however long they are comfortable with it. My 9 yr old ds2 and 5 yr old dd sometimes bathe together and when my 9 yr old niece stays, she also bathes with dd, their choice entirely. If any of them said they didnt want to then I would go along with that.
My two DDs are 6 and 10. I think it was a couple of years ago that I stopped bathing them together, because it wasn't easier to keep them together. DD1 was 7 and there wasn't really room for them to comfortably have a bath (they both like to lay flat to get their hair wet).
As a compromise, because DD2 still liked to spend time in the bath with her sister, DD1 would have her bath, then DD2 would join her, then DD1 would get out to get dry while DD1 stayed in to have a play. It worked well for us.
Now they have completely different baths and the bit where they were both in the bath doesn't happen any more.
I take them swimming regularly and they spend time in the water together like that now.
They don't row over space. I am calmer because I don't end up shouting when they get carried away splashing in the bath and my floor and wallpaper remain drier.