There is no definitive age at which a mare is classed as too old to breed. As a mare ages beyond her early/mid teens her chances of conceiving and carry the foal to full term start to decrease. There is some evidence that a mare having no previous pregnancies (or too many pregnancies) can reduce the chance of a mare conceiving but this change is fairly small. There is also evidence that a mare of 18 years + produces more abnormal eggs than younger horses which will lead to early embryo abortion and have more chances of inflammations, infections and contamination. I would recommend a full health check by a vet who will be able to advise you on her suitability to breed.