It is so sad when a mare goes out this way, but as to your question, I am afraid it is idiosyncratic. Every mare is different and has to be evaluated for herself and her condition. Usually an older mare tells you when she is ready to quit by not getting in foal, or slipping early, a couple years in a row. If she continues to conceive easily and deliver without complications, an older mare can produce into her twenties. [I have an old friend in Tennessee who had an unregistered TB mare who kept popping them out into in her mid/late 20s - catch on the first cover, become a barrel on legs, deliver before you know she's ready - that was just her.] Other mares are ready to quit by 15. I do think one ought not to try to drug an older mare into conceiving after she's told you it's her time, economic considerations notwithstanding.
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