Phineas was done at the age of three, and Henry at 11 months, both at the shelter, so I'm not sure, but I do know that the local Humane Society microchips all animals before they're adopted, and from time to time they adopt out kittens as young as eight weeks. Might vary, though, like how some vets will neuter as young as a few months, other not until after six months. Oh, and it's quite expensive here - I think my vet wants $80 for it, which is why Meeko and Essie haven't been done yet.