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Something a mod said in a thread where he answered questions about the rules of /e/.
So, a good way to determine if a girl is too young is by looking at her hips. Even if the girl has flat chest, but has wide hips (like ishikeis or bosshis small breasted girls do), its not too young. Not many elementary school girls have wide hips.
And the pictures of girls with narrow hips (go to gelbooru and look up for Focke Wulf artist, most of his pictures qualifies as "too young" imo. but I personally still like his pictures) are usually too young looking. If a girl doesnt have matured hips nor breasts, she most likely hasnt hit her puberty yet.
Thats how I determine if I can post a picture to 4chan or not.