Kittens should be neutered at about six months of age, before they reach sexual maturity. Neutering a safe, inexpensive and permanent procedure, not only prevents accidental breeding, but also reduces behavioral and health problems.
Our "Take Pity" vet, Dr, Sharyn Jones, says when considering neutering feral cats, younger is better. If they are at the age where you see them during the daytime (mama doesn't have them hidden away), then they are old enough to bring in for the procedure. This is also a good time to try to tame them if you are going to get them "off the streets" and into a good home. Meow!