AERC’s rule 3.1 explicitly states, “The equines entered in distances of 50 or more miles must be at least 60 months of age at the time of the ride with the exception of those entered in single-day 100 mile rides. Equines entered in single-day 100 mile distances must be at least 72 months of age at the time of the ride.”
For limited distance rides, rule L2.1 notes that the age requirement is a minimum of 48 months. For introductory rides, the age requirement is also a minimum of 48 months.
Age is determined from the actual foal date. That means you cannot enter an equine in a ride even a day shy of his actual birthday.
Underage horses are automatically discovered when ride results are entered into the AERC database, and the rule infraction is sent to the Protest & Grievance Committee for resolution when the period underage is more than 30 days. For violations less than this period, the horse is marked Disqualified in the records and the owner is notified that a second violation will will be sent to the Protest & Grievance Committee.
If you register your equine with AERC, be careful not to ride him in an AERC event until he is old enough — 4 for limited distance rides (less than 50 miles), 6 for one-day 100 mile rides, and 5 for other endurance distance rides.