unless Hagiwara Mai gets one within a year( she's 11 yrs old), risako is holding that record rite now. and it seems H!P isn't accepting applications from elementary grade students anymore, so she like the youngest we've got.
i'm just saying.... i don't even know if any of the H!P Eggs is younger than her, but i don't think they'll have any solo pb for themselves anyway, so looks if not hagiwara, then risako.