You can get them fixed very young, as early as 8 weeks but I think it is best when they hit around 4 pounds. If they are small, then around 10 - 12 weeks should be fine. It's easier on them although the vet needs to be a little extra vigilant about anesthesia. A baby needs much less than an adult of the same weight i.e. adult drug dosages must be scaled down to account for immature kidney/liver function. And since most of the time spays are done under general anesthetic with isoflurane or sevoflurance, the kittens are difficult to intubate if too small.
Remember, some early maturing kitties can get preggo at 4 months! That's 16 weeks to you and me
but a teenage pregnancy to the cat. It's easier on the cats if they do not have their first heat also. We always charged extra for females in heat since it was always a bloody affair and more of a risk to the cat.
As far as males - eh. If you can feel them, you can yank 'em.
Nothing has been proven that male cats getting neutered at a young age causes problems with urinary blockage and such. Neuter them ASAP! It's done under a heavy chemical sedation with pain relief so it's an easier operation. The younger they are less nerve/blood supply there is so as they get older, it gets ouchier (medical term
) Plus, they don't get in the habit of spraying.
Hope this helps!