I don't know about AMHA, this is the rule for the AMHR. The rule is in a supplement from 2009, (on pink paper), on page 5.
Youth 13-17 year olds may show senior stallions, (3 years or older), in youth or open hunter, jumper, obstacle & versatility. Youth 13-17 years of age may show senior stallions in open halte and performance classes. Youth are not allowed to to show a stallion in any youth halter or youth showmanship class with the exception of youth 13-17 may show senior stallions in yuth or open driving classes. Youth under 13 may not show stallions of any age, including junior stallions, (weanlings, yearlings or 2 year olds).