Well, it's not that unlikely, to be honest. I wouldn't be surprised if at least someone hit 26 before graduating. But I'm sure that the next 5 years will be filled with graduations. Yes, also older ones.
I always wonder why girls stay idols. Don't get me wrong, the income is decent, they get to perform and do idol stuff, but at some point they can't grow anymore. That's why it's called a "graduation", not just "end of contract". Honestly, at some point in life I think being an idol actually keeps you back more than allow you to grow. You can't love, your timetable is strict, it's very likely that some wota talk about every little thing you do. I honestly don't get why you'd want to be an idol over the age of 20, let alone 25.