I know i should know the answer to this. Age must be catching up to me as i can't seem to remember things if i don't write them done. I am thinking that the age was 18 , but not sure if that is for a mature stallion.
Pg. 242 of 2008 rulebook-Youth 13-17 may show senior stallions(3 yrs or older) in youth performance classes. A Youth 13 years of age or older may show a stallion in Open Halter and Performance Classes. Youth 13-17 may show senior(3yrs or older) stallions in Youth driving classes. Youth under 13 may not show stallions of any age, including junior stallions(weanlings, yearlings, or 2 year olds).