The recommended age for spaying a female dog or castrating a male dog is between 6 and 9 months. However, no large studies have been undertaken to determine the optimal age for the procedure. This age is just the normally recommended age for sterilization to occur.
The age recommendation first became the norm following World War II, when more families had the money to afford the procedure. Surgical techniques at the time and anesthetics available dictated that the animal must be a minimum of 6 months old in order to be sterilized. However, dogs can be spayed or neutered at nearly any point in their lifetimes.
In years past, it was believed that allowing a dog to have a litter or a heat before spaying her was the most prudent thing to do. However, female dogs who have been allowed to mature sexually have an increased risk of developing mammary tumors. After two heats, the risk is as high as 26 percent. Removing the dog's gonads before she has a heat reduces her cancer risk to 0.5 percent.