From r/generationology

1997-1999 borns are not Gen Z

They are not true Gen Z. They were in high school during the early and mid 2010s which is was dominated by millennial influence and attributes. Nobody even thinks anybody who graduated in the mid 2010s are Gen Z. People would find anyone who graduated before 2020 to be millennial which is one of the reasons. Also, 1997-1999 borns didn’t experience smart technology as people born 2005+. Their first phone was a flip phone or they gotten the early smart phone models. 1997-1999 borns arnt considered iPad kids which is something that defines Gen Z. They were too old to be considered that. They grew up with analogue technology and then on their teen years, they saw the transistion to digital technology. This greatly separates 1997-1999 borns from Gen Z . This would actually move the transition generation.

2000-2003/4 borns are the true zillenials. Not only were they the last to experience and remember full fledged analogue technology, they are the last to have any sort of millennial influence and attributions. They also graduated in the late 2010s and early 2020s which marks the transition to millennial and Gen Z.

2005 borns would be the first to be Gen Z because they are the first to be the optimal in technology usage. People even notice the differences from people born before 2005 and people born on 2005 and onward.

In conclusion, that is why 1997-1999 borns are not Gen Z. And why Gen Z should begin in 2000.