I figure Sanan-route Chizuru is the oldest and Okita-route Chizuru is the youngest. If Sanan-route Chizuru is older that would mean she probably worked with her father on developing the Water of Life, although clearly she didn't know what it was or how it would be used. Which I think could also explain why she ends up helping the Furies and not just her love interest in that route - she carries more responsibility towards them. There's possibly also an intriguing possibility of her being asexual-but-kinky, if she never had any interest in relationships before but *this* is what gets her interested. (Why yes I have overthought this.)

Chizuru would still have to be somewhat younger, since he does call her a child as a way of discouraging her crush. But maybe not so much younger.

I did the Sanan route third, after Okita and Heisuke, and while I *loved* the inversion of Heisuke's route I didn't have the experience of watching Sanan be awful over and over. I'm sure that made a difference.