All of the characters in Ichigo Mashimaro except for Nobu are 10 I believe.
Satoshi, Ritsu's brother, from K-ON is only 14 by the end of Ritsu's high school career I believe. They never showed him in high school anyways.
Kana-chan and "Chief" from Kanamemo are younger than 14. Kana's a middle schooler and Chief is in elementary school
During the 1st season of YuruYuri the characters are 14 or younger. I think a new school year started with the second season but can't remember. In which case Kyoko, Yui, Ayano, and Chitose would be 15 depending on birthdays.
Kana and Chiaki are younger than 14 I think from Minami-ke