@goodie-bag Middle school age isn't really that uncommon in Japan. High school age is rarer but does occur. If it's remaining non-sexual it's not really going to be any weirder to the ones participating than it was before some age cut off point. The factors I'd say contribute to it stopping are generally going to be your basic puberty embarrassment coming in to play, or if you're not quite there yet the other kids saying it's weird because their parents or siblings told them it was weird after they were either told by other kids, siblings, or parents in the past that it was weird and so on and so forth.
Weird's subjective, y'know? Where I live we already think it's weird for any of that past a much earlier point. I don't think I have any memories of my parents actually bathing with me, so much as perhaps washing my hair or just keeping an eye out so there's no accidental drowning or something. Fully clothed, they bathed separately before or after.