And thus we find that this is a light version of Onee-san wa Joshi Shougakusei ni Kyoumi ga arimasu.
Kasane goes for a certain type of young teen girls, and when they grow up and don't fit the type anymore she unceremoniously dumps them. It's what happened to poor sweetie Mimei (who is still disgruntled about that).
And Meguru's role in the story is Coco's: she's the one who will win Kasane's heart forever and make her discard all her superficial prejudices about age and looks!