Horse breeders want foals out of proven, producing mares. Naturally, mares which have had several good foals will be older. Older mares get pregnant as readily as younger mares, but they do not stay pregnant. There is a higher incidence of early embryonic deaths in older mares.
Older mares have older ova which are more susceptible to early embryonic death.
The uterus of the older mare seems to be capable of providing an adequate environment for a fetus. Older mares must be in good body condition. But, it is not wise to get older mares fat. The extra weight could negatively affect their movement, especially if they have arthritis. Horse breeders should have a uterine biopsy taken of older mares, which is a good predictor of a mare's ability to stay pregnant.