An immature rabbit will have structures that are not well developed, making the surgery more difficult (and in males the testicles do not even descend until about 3 months). You should consult with your vet about the proper age to do your particular rabbit. With males especially, your vet should be able to tell from a quick examination whether your bunny is ready by the presence or absence of testicles in the scrotum, but a general exam can give your vet a good idea about the maturity of your rabbit and readiness for surgery. If your rabbit is 4 months or older and exhibiting behavioral changes that include increased aggression or marking behavior, he or she is ready to be neutered or spayed!
Related questions:

