If you are in close collaboration with your vet regarding your 21 year old mare, there is nothing wrong with trying. As long as you are aware of the problems that can arise and take all the necessary precautions to allow her to have a successful pregnancy. You may have a heck of a time getting her in foal, or keeping her in foal but that's the price you pay with older mares.
My oldest is my 11 year old. It's not that old, but she's still a maiden.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."