It’s in the name. If you were a kid in the 90s, you’re a 90s kid. Babies are not the same as kids. Kids can cover a wide range of ages, as they can be referred to as that their whole life with a certain kind of phrasing.

I get the line is debatable, but I feel like consistent early conscious memories is about the starting line for “kid”. I’ve never really heard of it being used to mean when they’re born in conversation.

This is just my experience, but I’ve never heard of this particular molehill.