A real range there then. I guess it depends on gender quite a bit.
My DS and DD1 were about 6 and 4 when they stopped because DD2 had come along and DD1 loved having baths with her. They had baths together until about 10 and 6 they had always enjoyed it but DD1 definitely needed more space. She's 14 now
DDs 3, 4 & 5 were only 15 months apart, then twins and used to have all three together until a few months ago as space was really becoming an issue. They are 6,5,5 now and any 2 of them still have baths together, especially the twins. Very occasionally they still have all 3 together as a special treat (they love it) but that's quite rare.