I think the best marker for what 'decade' someone's childhood exists in is to have people list the media that stuck with you as a kid.

I see the '90s kid' cutoff being the prevalence of Spongebob Squarepants, it aired in 99, so kids hit their formative years watching it if they were born early 90s and beyond making them '00s kids'. It's like stratigraphy for media. Actual 90s kids were already too old to adopt him en masse so their media was Ducktales and Rescue Rangers, preceded by the generation raised on Bugs Bunny before they removed the bigotry, and the generation before that Mighty Mouse and Pink Panther.

As usual there is always some bleedover, for example I'm part of the beginning of the Ducktales era yet still have a great fondness for Pink Panther which was already in reruns before I was born.