We haven't had many really "OLD" mares......yet. But the few we've had -- believe it or not, TELL us when they are ready.
Right now, I have one special girl who is 16 years old this year. Technically that is not old -- more like "middle age" but I know she's ready to just be a Grandma/Nanny. She is carrying her last foal right now.
Then, in another pasture I have a 20 year old lady who would KILL me if we retired her! She LOVES being a mom!
What's important is how healthy the mare is and if THEY enjoy it.
MA