Okay, this is a topic I know a little bit about, because I'm often the person who has to take our shelter kittens to be spayed/neutered (I can't do the regular feedings or cleaning, because more than 2 cats at a time give me asthma attacks, so I do stuff like ordering food, begging neighbors, students, colleagues, etc., to adopt, or sponsoring special-needs cats), and we use four to six different vets (usually), since everybody has their "favorite" vet, and each vet is usually willing to do a certain number at the lowest possible price. The "usual" age is six months in Germany, but the veterinary schools are beginning to "rethink" the matter, and to recommend the procedure at an earlier age, i.e, two to three months. I've asked several vets about it, and all of them have told me that they neuter at six months because they weren't taught how to deal with younger kittens, as anesthesia in particular can be tricky. And they said: 1) The procedure is easier with younger kittens, because they have less body fat, and 2) Younger kittens recover faster. In other words, the vet's training plays a major role. The younger the vet, the more likely she/he will neuter kittens at a young age.
fwan, kittens definitely don't stop growing when they're neutered. In fact, males castrated at a young age are more "elongated" than those castrated at a later age. I know I have a link on that (in German) bookmarked, but I'll have to search for it.
Oh, as an added note, those of you who've seen my thread about the hare-lipped kitten in "Health & Nutrition" might be interested in knowing that Momma cat has been examined by two vets, and both estimated her age at 4 to 5 months!