>>9697251330 years old is a perfectly fine age to be, it's just that idols are expected to be within the 16-20 range, hence why Marine gets called a hag despite being not that old - at least compared to Korosan. Speaking of which, Korosan is good at hiding her age and doesn't get called a hag by her fans since she doesn't play into it like Marine does. The only thing you can go off by is her taste in older anime and games.
Also they're all Japanese! I literally don't know why they care so much about being old when they can literally look like they're 23 even if they turn 40 at some point. East Asians are vampires, man.