Care Animal Hospital recommends spaying or neutering puppies, kittens and bunnies between 4-6 months, as long as they are over 2 pounds.  Current veterinary literature shows there are proven medical benefits to spaying and neutering at this age including reduced incidence of some cancers and infection.  While there has been some discussion about waiting until the pets are more mature for a variety of reasons, at this time, there is not definitive scientific studies showing a medical benefit to waiting.