It used to be 23 in many states before 1990, so I knew multiple people who would be 23-25 by the time they finally graduated. Many were held back in grade school (on purpose by their fathers), to get an edge in sports and used their siblings or cousin's birth certificates to keep playing those sports. Lots of them were Puerto Ricans playing American League baseball trying to break into the minor leagues before they embarked on a life of crime or installing cable lines.
One offensive lineman at a prominently black high school was supposedly in high school for 9 years, he transferred from a different high school after taking a year off for knee surgery to keep playing football and was 300 pounds, which was impressive in 1986.