There is no exact stated answer. Sorry. It is "suggested" that you assume 21 is the age to be on the safe side. Unfortunately the powers that be use the term "minor child" without providing an exact age.
See - http://www.cozumelinsider.com/Vacplan?From=Lodging
The text is copyrighted.
"Therefore, to be on the safe side, it is best to contact the airline in advance and err on the side of age 21 and under and being a "minor". As always, please email the Consulate using mercons@prodigy.net.mx or contact the U.S. Consulate in Merida, Mexico, if you have any further questions. "