Female cats: Should be neutered (spayed) at 6 months of age.
If female cats were left un-neutered they would continue to have at least two litters of kittens a year for the rest of their life.
Neutering also stops cats from straying.
Male cats: Should be neutered (castrated) at 6 months of age.
Neutering prevents them from fighting (resulting in bite wounds that turn into bad abscesses) and wandering off. It also helps avoid urine marking (spraying) which results in severe unpleasant smells in the house.