Some people are saying that kids are being held back multiple years. This is not the case (at least in Baltimore, Annapolis, and DC). The very oldest a kid can be as a senior in these private school leagues is 19.

MIAA (Baltimore/Annapolis) and IAC (DC/MD/VA) both say "A student who has attained his/her 19th birthday before August 31 of a given academic year is not eligible to participate."