The thought used to be that you let females go into heat once before neutering, and I suppose the equivalent for males. Not anymore. The thought generally is that what the cats have never experienced they aren't going to miss so much.
We at one time had a beautiful male Siamese. Unfortunately the poor guy broke his leg, and after the vet was done pinning it he told us he had thought about neutering him while he was already out, but he was such a beautiful cat he was afraid we might want to breed him. My wife and I had not even thought about it, but we started looking for a female. They were in short supply in South Dakota where we were at the time. We finally had a pet shop owner who got one for us from a relative in another part of the state. She was taken from her mother before she was really old enough, and then started having kittens when she was really to young. Our male was a big cat, and the female was always small, and was was not the best cat we have ever had when it came to the litter box. But she was a great mother. She may not have been the best herself in using her litter box, but she was great when it came to training her kittens. The Tom also made a great father. When mom wanted a break, he'd get in there and kitten sit. They would come and talk to us until we would come and take a look at the kittens. Actually, mom never talked much, but dad made up for it. She just plain never had much personality, but the male had enough for both of them. We finally sold the two of them, but the best of the first litter went to my parents, and we kept the best of the last litter. Ten years later we visited the little town where this had taken place, and where we had sold a fair share of the kittens. We were visiting with a couple when their cat walked through the room. It was obviously not a purebred Siamese, but it also very obviously had some Siamese traits. They said every cat in town now had some Siamese in it.
Anyway, if this has a point, its that many vets are now recommending neutering take place much earlier than they used to.