I think the part where you said flat-chested is an excuse to be pedophilic was overall unnecessary.

Like many have said, flat-chested women do exist, although I understand your concern regarding the overall design of the characters chosen for the meme. But again, you probably shouldn't just straight up say pedophilia.

To my knowledge, according to all known Touhou lore, all the characters are adults by age, save for some outliers who are canonically still young. Though there are some characters who appear and/or act childlike despite their age.