My Mum said our Dad (3 girls) started getting uncomfortable when we (all 3 girls squeezed into a rushed shower nightly) would catch the drips off the end of his doodle (?) in a cup & splash it on ourselves!!!!
He decided then that we should probably bath, I am the middle sister & I think I was around 5ish (I cannot remember it tho!) My older sister would have been 6+ & youngest would have been 3ish!!!

My sister stopped bathing her son & daughter together when it was school holidays after he had just finished a year of prep & sister was 3. As she thought starting grade 1 & still bathing with sister probably wasnt cool!