I wouldn't consider people born in 1999, '90's kids' a 90's kid is someone who was a child during the decade and was old enough to experience the different aspects of the era.
Regardless yeah, it's crazy to think that by the end of the year a large majority of High schooler's would have been born in the 2000's. My little brother was born 2002 and he's a freshman
I turned 20 last April and I just feel so much more grown. It was amazing and scary experience all at once.