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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.
They have to be at least 8 weeks old to be spayed or neutered. It doesn't have to be done so early, but it should be done sooner rather than later, as studies have shown that cats that are spayed/neutered later are at greater risk for certain serious conditions and diseases, such as breast cancer.