That comment is basing its entirety off a shitty UK parenting magazine survey to make it's claim about childhood.

If your childhood ended at age 12, then that sucks, I guess. The legal definition is until 18. So is a shitty UK magazine the authority here, or is the US law?

Probably the actual law.

So with that settled, someone born in 1978 would have been 13-17 during the 90s.

Kids born in the 80s would have been 10-17 in the 90s.

Kids born in the 90s would have been a maximum of 9 in the 90s. Hardly old enough to have knowledge of the 90s culture.

80s born humans are the primary 90s kids.

It's "90s kids", not "born in the 90s" kids. It's written that way specifically.