Surgical sterilization is surgery requiring anesthesia. Because older pets may react negatively to anesthesia, there should be caution when altering, although the benefits of spaying and neutering in general far outweigh the negatives.
If the optimal “window” for altering the pet is missed, the surgery can still have beneficial effects up to a certain amount of years. Dogs and cats do not experience “menopause” like humans and can reproduce throughout their life span. However, they do become less fertile as they age. Often, by this time, unspayed females will have experienced complications due to the fact that they are unspayed.
Depending on the animal’s age, it still may be beneficial to have it spayed in the later years. Discuss your concerns or questions with your vet.