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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.
Studies have shown that kittens can be spayed and neutered as early as 8 weeks of age, with no deleterious effect. Your cat, at 16 weeks of age, can be spayed with no worries at all.