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While spaying is traditionally recommended for kitties between 4 and 6 months of age, spaying as young as 8 weeks old is considered safe, according to Cat Channel. Many animal shelters actually spay kitties prior to adopting them out at this age.
You can spay/neuter your kitten once they get to 2 pounds. However, most vets would recommend waiting until they are about 3 - 4 months old but prior to 6 months old. You do not want to wait too long. Spaying/neutering your cat early will prevent it from developing spraying habits and aggressive behavior.