As a half-youkai, I don't think Rinnosuke refers to women by their actual age, but by their appearance. When he ?first? meets Yukari he describes her as a girl (少女). From then on he continuously describes her as 少女, even after knowing she's lived much longer than he has. As an anecdote, when Rinnosuke attempts to smash the computer for being ~ominous~, Yukari's hand pops out and stops him. He doesn't know it's Yukari, but describes it as a slender girl's hand (か細い女の子の手).
It would make sense that if you lived a long time, you would just describe people as they seem. The difference between "girl" and "woman" might become pretty arbitrary and confusing otherwise. What's interesting to me here is how Rinnosuke's description might differ to someone else's, possibly more recent, description of Yukari.
It's hard to get a good reference frame because it isn't often we have actual first-person descriptions of other characters, besides in CoLA.
