Does that mean they took an 18 year old and put him back in class with 15 year olds expecting him to stay there for four more years from 19-22? That can’t be right. What does sent him back to the 9th grade mean in this context?
State by state data: https://nces.ed.gov/programs/statereform/tab5_1.asp
Texas allows up to age 26...heh.
It's possible at age 22 the state is not required to provide him an education, but chooses to do so anyways
Here's Texas, for example: https://statutes.capitol.texas.gov/Docs/ED/htm/ED.25.htm