An owner might test breed a few proven mares to a good stallion prospect when stallion prospect is 2 or 3 years old, and then continue on showing and promoting the stallion prospect-- if the "test crop" is not up to par, the prospect can be gelded before alot more time and money is invested into a show career and advertising as a stallion prospect-- if the test crop looks favorable, the stallion's career and promotion can continue and there will also be some early offspring growing up that can begin their show and competition careers, and show prospective mare owners what the stallion is siring.
Mares are capable of being bred at 2 or 3, but most people wait a few more years. Some people want the mare to have a successful career to prove she is worthy of reproducing-- others are OK with the mare having proven close relatives.
Breeding always has risks to the mare, and some risks increase as the mare gets older, but with proper care and feeding and appropriate vet exams and care, it is not unreasonable to see mares producing into their 20s.
Edited to add-- colts and fillies are sometimes physically able to be bred as young as 6 to 10 months-- but obviously just because their little bodies are capable does not make it the best choice. 2-4 years is considered the youngest to "safely" breed, with 2 not being acceptable to many people as still too young.
Last edited by Eastowest; 02-07-2010 at 10:35 PM.