I don't really want to turn this into an essay, but here goes
Morning Musume isn't entirely comprised of U-18s: the eldest, Takahashi Ai, is 23 (and a stunning, ravishing beauty at that); "subleader" Niigaki Risa is 21; (cutie-pie) Michishige Sayumi is also 21, and so on.
That said, Eri Kamei auditioning at 15 is far from the youngest recruit out of H!P: C-ute's Hagiwara Mai auditioned (and passed) at 6 back in 2002.
The "junior" groups within H!P would be S/mileage, C-ute, and Berryz Koubou, each of which is, yes, stocked with teenaged members.
Even so, the "senior" group has never shied away from younger members: Kago Ai and Nozomi Tsuji were both 12 when introduced, and the youngest current member, Mitsui Aika, is all but 17.
As for the legality of H!P recruiting and promoting such underage girls: I'm sure someone from the legal dept. cleared it years ago.
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