The authors of the next 2 articles studied long-term effects of pediatric gonadectomy and concluded that gonadectomy can be safely performed in cats under 6 months of age. These articles are followed by reports of 2 studies of cats neutered at over or under 24 weeks of age. The authors found no increased risk of disease or other health problems in cats neutered early.

Long-term risks and benefits of early-age gonadectomy in cats. (PDF, 158 KB)

Highlights:
   Studied 1,660 cats adopted from shelters
   Early-age gonadectomy beneficial in male cats
   Asthma, gingivitis decreased in both sexes
  Gonadectomy safe in cats under 6 months of age

Results of that study also suggest that prepubertal gonadectomy may result in some unidentified protective effect on the urinary tract, compared with traditional-age gonadectomy in some cats.

Long-term outcome of gonadectomy performed at an early age or traditional age in cats. (PDF, 270 KB)

Highlights:
   Health status of cats monitored for 3 years
   No increase in physical or behavioral problems
   No increase in infectious disease in either group
  Early-age neutering did not increase problems

All this information was taken from  American Veterinary Medical Association’s website.