Fillies become sexually mature at around 18 months old, and can foal as two-year olds. However, they are still growing at this age, and pregnancy may hinder their growth. Ideally, mares should not begin breeding until four years of age (to foal at five years), although some are put in foal when they are three.

It can very much be dependant about the maturity and breed of the horse. It is certainly possible to breed a regular horse at the age of 2 for a foal at 3. That is if the filly is mature and on a good plan of nutrition. She needs to not be too fat or too thin and should have a good source of free choice minerals, high quality hay and grain as needed. You have to remember that being in foal and growing at the same time put additional stresses on the filly.