"older mares might begin cycling later in the spring than younger mares, and the time between ovulations might lengthen. Problems can arise with ovarian follicle development, resulting in a longer time in the follicular phase and smaller follicles from a slowed growth rate.
It is not uncommon to see mares above 20 years of age that take three to four weeks or longer to build a follicle suitable for breeding.
McCue says the incidence of ovulation failure increases with age. For young mares, the dominant follicle (the one that grows fastest in preparation for ovulation) fails to ovulate in less than 5% of cycles. But when mares are 15 and older, that rate can increase to 13%.
“Some older mares have very irregular cycles,” says Carlos Pinto, MedVet, PhD, Dipl. ACT, chair of ambulatory medicine and theriogenology at Tufts University’s School of Veterinary Medicine, in Woodstock, Connecticut. “It is not uncommon to see mares above 20 years of age that take three to four weeks or longer to build a follicle suitable for breeding. Even if they ovulate, there is a question of how normal the corpus luteum is to produce adequate amounts of progesterone to maintain a pregnancy.”
https://thehorse.com/148754/breeding-older-mares/