This lists the "maximum age limit to which free education must be offered" which may not be the same as the "maximum age limit to which free education can be offered"

It's possible at age 22 the state is not required to provide him an education, but chooses to do so anyways

It's a little hard to summarize as the rules are complex and different for each state.

Here's Texas, for example: https://statutes.capitol.texas.gov/Docs/ED/htm/ED.25.htm