You must be 19 years old by the end of the calendar year of the draft , anyone who starts their NBA career at 18 would be 19 on or before December 31. Also your logic is backwards, if he were in high school as you say he should be then he’s already 18 and would be 19 before playing a game in college his freshman year which would be next year.
His birth date is October 14, 2006.
Jayden Quaintance is an example of a player who should be a true senior in high school. His birth date is July 11, 2007 and he isn’t draft eligible.