I would check her foaling history - if she has missed in the last couple of years that MIGHT be a warning to you. If she has been a consistent producer and foaled as early as 2010 then she is probably a gold mine. The key to some of these older mares is KEEPING THEM PREGNANT. I know that sounds cruel, and anyone who knows me that I am the biggest crusader for the mare out there - but if she is keeping her weight up, foals with no problems and is a happy pregnant mare, then the best thing for you to do to ensure that she will continue to produce is to keep her in foal and PANDER TO HER NEEDS. If she is looking worn out, gets ultra thin, has trouble foaling, etc, etc, then it is probably time for her to retire. I have several mares that are in their late teens and early 20's that are happily perking right along - rest assured that when they don't have an easy pregnancy that they will retire to a cozy stall and live out the rest of their lives as pampered pets.