I don't have a max age. I think as long as the mare is in good condition physically and reproductively age is less of a factor. That being said I would evaluate each mare as an individual and evaluate my reasons behind wanting to breed an older maiden mare. Is she exceptional in some way performance or pedigree that would warrant taking that step/risk.
"I would not beleive her if her tongue came notorized"
I bred my maiden mare at 11. She had cycled well and is due in April, she's not had any issues to date (keeping fingers crossed). My vet said that he'd consider mares up to the age of 15 if they were in good health and cycling well, much older than that and he said you start to run into more issues, not impossible but the chances for problems begin to increase significantly.