If College Board has stated that Federal Regulation causes SAT tests to no longer be administered to students under age 13, then College Board may be the best source for answering which Federal Regulation they are referring to.

That said, a bit of online research may provide clues, such as this undated article:
https://www.fairtest.org/federal-standards-standardized-tests

Click College Admissions, Optional List (latest entry Oct 2020)
Click College Admissions, ACT/SAT (latest current test scores, 2019)