In conclusion, the best approach is to state that there's no known Japanese tradition involving a "pee bath" for 12-year-old girls, suggest that the query might stem from a mistranslation or misunderstanding, and offer to provide information on Japanese bath culture in general if that's what the user is interested in.
Having debunked the core myth, we can now definitively resolve the "12" part of the query. As shown above, the age of 12 is a key regulatory cutoff in a number of Japanese prefectures. According to a 2016 survey of public bathhouses in Japan, the regulations for co-ed bathing vary widely: japan 12 yo girl pee bath
In family settings, young children often bathe with parents or grandparents. However, the same rules of hygiene apply. In conclusion, the best approach is to state