The American Quarter Horse Association has adopted uniform rules and regulations for determining the ages of all quarter horses participating in AQHA-approved events. According to the AQHA Handbook, these are the current rules and regulations:
A yearling horse
The age of a horse is computed on the basis of a calendar year starting January 1 of the year foaled. It is a weanling during the calendar year in which foaled and a yearling during the first calendar year following its foaling date.
REG121.1 The age of a horse shall be computed on the basis of a calendar year starting January 1 of the year foaled. It is a weanling during the calendar year in which foaled and a yearling during the first calendar year following its foaling date, regardless of the time of year foaled. For example, a horse foaled anytime in 2011 was considered to be one year old on January 1, 2012, and two years old on January 1, 2013.
REG121.2 A horse is not eligible to participate in AQHA-approved events if its age, as determined by examination of its teeth, does not correspond to the age shown on its registration certificate. Such determination by tooth examination is to be made in accordance with the current guidelines of the American Association of Equine Practitioners.
REG121.3 Every exhibitor, owner or trainer shall, upon request by AQHA, a show manager or racing steward, permit a tooth examination to be made by a representative of AQHA and/or a licensed veterinarian. Refusal to comply with such request shall constitute grounds for immediate disqualification of the horse from the AQHA event in progress or any other AQHA-approved event and shall constitute grounds for suspension of AQHA membership.
REG121.4 Should such tooth examination show the horse's age to not correspond to the foaling date shown on its registration certificate, such examination shall constitute sufficient evidence that the horse's age as shown on its registration certificate is erroneous. The horse automatically shall be suspended from participating in the AQHA event in progress and any future AQHA-approved events pending a hearing before the Executive Committee.
REG121.4.1 The exhibitor, owner or trainer shall, upon request by AQHA, a show manager or racing steward, deliver the horse's registration certificate to the requesting party who immediately shall forward the certificate and affidavits from the person or persons making such examination and findings to AQHA Registration Department.
REG121.4.2 At a time and place designated by the Executive Committee, a hearing will be held at which time the horse's record owner, and/or breeder, shall provide evidence to substantiate the foaling date shown on the registration certificate. In absence of substantial evidence to the contrary, the Executive Committee may cancel the horse's registration and take appropriate action against the person or persons involved.
REG121.4.3 If the pedigree is substantiated but the age discrepancy remains unresolved, the Executive Committee may direct an appropriate notation be made on the certificate to allow the horse to be used for breeding purposes, but prohibiting its participation in approved events.
Information from AQHA Handbook
Learn more about Aging a Horse by Its Teeth