A horse's reproductive system consists of the ovaries, vagina and uterus in females, and the testes and penis in males, according to Merck Vet Manual. It functions similarly to other mammals in that the ovaries produce an egg that is fertilized by sperm, and the offspring grows in the uterus.
Females become sexually mature at about 12 to 15 months old, and males between 12 and 14 months, according to EquiMed. Most horse owners prefer to wait until the filly or colt has stopped growing at about four or five years old. Mares can successfully give birth well into their 20s, though fertility begins to decline at about 16 years of age.