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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.
Most kittens can be safely spayed or neutered at the age of six weeks, by a vet familiar with early spay/neuter techniques. If you have no such a vet locally, consider asking for a spay/neuter deposit, to be refunded when the adopter of a kitten presents evidence the S/N has been done.