Age-Old Question: How Old Is Too Old To Breed A Mare? by Paulick Report Staff|04.15.2019|8:15am Generally, mares are past their reproductive prime when they reach 15 to 16 years old. There are mares that can carry and deliver healthy foals into their 20s, but the majority of them have had multiple successful pregnancies before. To see if a mare can be bred, a veterinarian should perform a breeding soundness exam (BSE); a rectal palpation and ultrasound will inform the vet of the health of the mare's uterus and her ovaries. Some mares experience decreased heat cycles as they age; the exam will detect any abnormal structures. The vet may also suggest a uterine culture and biopsy; a culture will indicate any infection that would decrease a mare's fertility and a biopsy would show the details of the uterine lining, possibly detecting abnormalities that can't be felt on palpation. Read more at Horse Illustrated.