Not maiden, but my mare had a huge filly this spring at age 18. I would say early or mid-teens if the mare is healthy, but I personally wouldn't risk it unless their were very valid reasons for breeding her; which on most occasions their aren't. But I suppose if there was a mare who was shown, and did very well, very consistently then maybe retiring as a broodmare would be plausible. But I'm talking higher-end mares, not your basic weekend warrior. I have mare in her 20s, and one who is 16, and while I've toyed around with breeding them in the land of the hypothetical, I've come to realize that those two probably don't need their demon spawn running around, nor could I do without my two best weekend warriors if something happened to them. (Among many other conformation-related weaknesses.) If someone is going to breed a horse that age, I certainly wouldn't take any shortcuts when it comes to their health.