Your gorgeous mare may have the temperament, conformation, bloodlines and talent to warrant breeding, but her age is an important consideration too. While it’s not uncommon to find mares bred during their 20s, fertility and foaling rates decline after 14 to 16 years of age. Pregnancy losses during late gestation increase in older mares and failures during early pregnancy are even more common.
Source: “Reduced Reproductive Efficiency in the Aged Mare: Role of Early Embryonic Loss,” B.A. Ball, University of California, Davis.