None of the OG quitted because they felt "old" though.
I know. They graduated because it was "time to move on," which either they thought about it themselves or UF suguested them since they reached adulthood. I didn't explained that part very well, but what I meant was that it just takes some girls longer to get to that point. It just came early with Kanon because she already felt "old" for a while.
She's just.... really bratty, which she apparently fulfilled a stereotype of an only child. She may seem immature about the situation, because we all know 20 is still very young, but she just felt like her time is now.
I currently don't know any other member that has a similar personality as Kanyon. And there is a lot more members striving for 10 year or up to age 25. But, with ANGERME turning into a rotational group and Hello! Project have a lot more units/members, the older members will eventually be pulled out. But, you know, we should enjoy the current members while we still have them.