The Procedure

Female dogs and cats are spayed by removing their reproductive organs, and male dogs and cats are neutered by removing their testicles. In both cases the operation is performed while the pet is under anesthesia. Depending on your pet's age, size, and health, he or she will stay at your veterinarian's office for a few hours or a few days. Depending upon the procedure, your pet may need stitches removed after a few days.
The obvious reason spaying and neutering is so critical is to prevent unwanted, accidental pregnancies. There are many more benefits, though, that are good for the pet as well as the guardian.

Minimum Age
Spay/neuter can be safely performed on animals as young as 6 weeks of age. Veterinarians practicing this technique report that the surgery is significantly quicker and easier to perform. Pet guardians report fewer post-surgical problems than those who have older animals altered.
Unaltered males are subject to a number of hormone-related medical problems as they age. They may develop prostate, perianal, and testicular tumors and cancers. Neutering greatly reduces the risk of these medical problems.