Technically you can spay/neuter from 3 months but there are arguments for waiting til they are 6 months old. My vet will not spey/neuter until the kittens are 6 months old because of concerns about recovering from anaesthesia, it is better for him because everything is bigger and because he believes it is better for the kitten.
However, even here in the UK, it is not unusual for breeders of pedigree cats to spay/neuter their kittens before letting them go to their new homes. Most of them do this because they sell the kittens as quality pets and not for breeding from. They also do this to protect the breed.
My two are pedigree but they came from a nice breeder who simply asked me to promise to get the girls spayed. I have heard of people who have promised to spay/neuter the kittens and then don't but I don't fall into that category :-)