around 10 weeks of age. the sooner the better.
Although some people suggest waiting until a cat is six months old, early age spaying and neutering are becoming popular. Physically, male cats neutered prior to puberty do not develop the large head and thick skin of intact males. Both neutered males and spayed females have a tendency to gain weight due to a decrease in roaming and other sexual behavior. However, weight gain can be prevented through dietary management.
Studies have shown that cats spayed or neutered at less than six months of age do not have an increased risk of physical or behavioral problems as compared to those that undergo surgery later. Early-age spaying and neutering may prevent problem behaviors before they occur. Spaying and neutering should also be considered for any pet with a behavior problem, regardless of age. For certain behaviors, surgery may reduce or eliminate the problem, even in older cats. Consult with your veterinarian or veterinary behaviorist for further information.
Male cats usually don't start "spraying" until 9-12 months of age. It is best to wait until they are 7-9 months of age to have them neutered, as new studies show that neutering them too young can cause urinary tract problems later in life. They are not fully sexually developed until that time! Hope this helps!