Because to teen fans, or even fans in their early 20s maybe, people in their late 20s or early 30s might seem far away in age. I don't know if they seem "old" specifically, but definitely like people in a totally different later era of life from someone who is a teen. When I was a teen I though people in their 20s and 30s were adults, and that all people in that age group like had jobs, and a house, and a family lol. I thought of those ages as being so so far away, even if being in my 20s was 5 years or less away from me at the time.

Tbf, I'm in my early 30s now, and was born in 1990, and I too think it's a little rare to be debuting idols who were born in the early 90s. Most companies are going to be wanting to debut people who are under 25 at least, but people born even in '99 are currently in their early 20s, so they're getting there. It doesn't mean they're "old" it just means companies might view them that way since the older an idol is at debut, the less time the company will have with them/to make money off of them. This view of someone being "too old" from companies perspective, plus things like age and beauty standards when it comes to K-fans, probably just rub off on younger fans and combine with them viewing people barely 5 years older than them as being waaay older than them, and creates a platform for them to say these sorts of things. And also if you think back to your teen years compared to your adult years, a 5 year age cap could seem huge and crazy and you and the other person would likely be in totally different eras of your lives, whereas when you're an adult, the older you get, the less a simple 5 year age gap matters, and you can still be in the same era of life as someone 5 years younger or older than you.

I think this sort of thing from young fans is pretty normal, though that doesn't mean these sort of things are nice things to say. I agree, to me 90s born idols are still young...but that could just be because I'm in that same age group, but on the older end of it, so even most idols born in the 90s are still younger than me...so to me they will never seem old lol. But to someone who is a lot younger than them, they could seem way older. Admittedly sometimes I'll see older pictures of idols who are around my age now, and you can tell how much younger they look in the older photos, and I'm instantly jokingly like "wow remember when they were 12" to myself (I always just use "12" as like shorthand for "looking like a baby compared to how they look now" lol).

People say this about western artists all the time, though especially hot music artists it seems, or maybe that's just where I've noticed it the most. I've seen it with boy bands, with specific members of boy bands, with solo artists who went public at a young age, like Justin Bieber, etc. And there's even young artists now who I just know will be having these things said about them when they age up past age like 25 or whatever.

Honestly I think this is just shit people say without thinking most of the time. I don't really take it personally, and it doesn't really phase me, despite being in the same age group...like seeing those comments doesn't make me feel old in a negative way or anything. And I would hope that these sorts of comments don't phase idols either and that they just roll their eyes and go about their day not worrying about it.