I'm pretty sure that they aren't, unless they hit puberty really late. Because I remember an episode of Higurashi where Shion or Mion said something about, "You'll find out when you hit puberty," or something along the lines.
I think its more like this because as KibahinaxSasuino commented, Shion and Mion say "You'll find out when you hit puberty" so that is why i think these are the ages Rena/Keiichi = 14-15 Rika/Satoko = 11-10 Shion/Mion = 16(maybe)-17