David Robinson was 6'6'' his senior year in high school. By his freshman year in the Naval Academy he was 6'9''. By the time he graduated he had grown another three inches to 7'. Some people just grow later, perhaps due to college providing a more regular training environment, better health services, and improved dietary options.
Didn't Ben Simmons say he grew a few inches in the past year? If it is true that he's now close to 7', that would be incredibly frightening for the rest of the league.