yeah i think calling yourself a "90s" or "00s" or any decade kid is kinda meaningless cause it has different meanings to different people. But its mainly used in that most of your childhood years were in the decade. If you had part of your childhood within the decade and you have actual grounded vivid memories of being a kid in it then i don't see the problem in claiming part of it if it's important to you.

I can only remember the end or a quarter of the decade so I can't possibly be a 90s kid. And most of what I learned about the world happened in the 00s. However, since I remember doing kids stuff, going to school and knowing some of the pop culture at the time I can say that it was part of my childhood