i'm not saying the industry hasn't debuted literal kids before but it wasn't a trend. during the 3rd gen the youngest idols to debut were around 16-17 and even that was considered too young. the only ones younger than that (that i can think of) are somi and yeojin from loona but they were exceptions.
now, the more girl groups debut the younger their maknaes become - you have idols debuting at 13-14 with no proper training whatsoever. during the 3rd gen you had idols training for 5+ years before debut and now them kids cannot even talk properly because their baby teeth are still falling out
and it's not like the age of kpop fans is decreasing either. yes, you have many people joining the fandoms but most of them are in their late teens or 20s. not to mention many 2nd and 3rd gen stans most of which are fully grown adults now are still actively part of the community and keeping up with the new groups.
the 4th and soon 5th gen are going in the wrong direction imo and all those stans calling idols in their early 20s "hags" for debuting so "late" aren't helping the case