Breeding older mares-what is your view?

I am getting a 20 year young mare, registered and very healthy, from a reputable ranch, that will be in foal.I have heard from both sides of the issue about the dangers of foaling for older mares, and just wanted everyone to fill me in on what they know about this- she is a quarter horse with buckskin blood that has been bred frequently with no problems at all in foaling. The owner has told me this horse's dam foaled great until she was 26, and died at age 34.Her sire just died at 36. Is it common for buckskins to be more hardy and sturdy than other breeds? Thanks!
kat