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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.
Cats can be spayed/neutered as long as they weigh at least 2lbs, are 8 weeks old, and are healthy. My friend does a lot of spay/neuter clinics in low cost areas and a neuter is extremely easy (it literally takes 10-15 minutes). I've had first hand experience seeing kittens spayed at 10 weeks.