Most broodmares are generally past their reproductive prime as they approach about 15 or 16 years old, writes Anna O’Brien, DVM, answering a horse owner’s question about a 20-year-old mare. She explains a variety of considerations beyond the mare’s health.
The best thing to do to properly evaluate the mare’s reproductive health and capacity is have your veterinarian perform a breeding soundness exam on the mare.”
Source: Horse Illustrated, February 26, 2019. Link.