Hitsugi and Chitaru are yuri characters. Chitaru's age seems according, but what about Hitsugi? She is really smaller than Chitaru.
Hitsugi and Chitaru are both at the same age, which is 15 years old. It is true that Hitsugi actually looks like a little girl, her height is also not up to her age, not even close, so it seems like Hitsugi is younger than Chitaru, but it's not.
Hitsugi doesn't even seem to have matured yet, which is the actual reason.
According to Akuma no Riddle AK, a social game which was released in 2012 (before the anime in 2014), Hitsugi and Chitaru were both 15 years old, even though Hitsugi was often to be mistaken as primary school student due to her short height.
Image courtesy of naomamayu's blog (Japanese)
*The game was shut down in 2015