Most animals are spayed when they are young. This is done in part because the surgery is technically easier. There are also health reasons as well as overpopulation issues playing into the early spay recommendation.
One of the things that can happen if an animal is not spayed is they can get a uterine infection. This can make the animal very sick and in some cases if untreated, can cause death. A sick pet is also a riskier candidate for anesthesia. And the infected uterus is fragile and can break open during surgery spreading the infection all over the abdomen.
Both cats did well after surgery. The cat with the infected uterus had to go home on antibiotics. She did not get too sick because her owners noticed the problem quickly. But her surgery did cost about three times more than the routine spay.