Traditionally, cats have been spayed or neutered at six months of age or older. However, many veterinarians recommend performing the procedure at an earlier age to further insure against unwanted pregnancies.
Spaying (ovariohysterectomy) is the surgical removal of the female reproductive organs (ovaries, oviducts, uterus). It is a recommended procedure for all female cats that will not be used in a breeding program. The removal of the reproductive organs eliminates the behaviors associated with the heat (estrus) cycle (i.e., kneading, howling, restlessness); greatly reduces the incidence of mammary cancer; and helps to decrease overpopulation.
Neutering (castration) is the surgical removal of parts of the male reproductive organs (testes, epididymis, parts of the vas deferens). The benefits, besides preventing impregnation of a female cat, include the reduction of excessive aggressiveness, urine spraying, and the pungent odor of intact-male urine.
And PLEASE keep your kitties indoors!