Depends if she has been bred in the past or not - if she has proven to be a good mother in the past, to carry and look after foals without any health problems, I can see no problem with breeding into the early twenties.
If it is a maiden mare, who is fully sound with no health problems, 16-18 for first foal maybe...
It depends on the health of the mare and the breeding also.
She's definately not too old atm - I prob wouldnt leave it past 16ish. They dont have a menopause or anything, but their ribcage becomes less pliant as they get older, so less room for foal inside her - so older mares often have a small first foal.