Kittens (and puppies) can be altered as young as 2 pounds or at about 8 weeks of age. This is known as pediatric spay neuter. (https://www.alleycat.org/the-wait-is-over-when-her-weight-is-over-2-pounds/)

While many of your typical veterinarians will recommend waiting until 6 month of age to alter the kitten or puppy, this is simply impractical since a kitten/ pup can become pregnant as early as 4 months of age. "Fix At Four!" Watch this cute video from Best Friends Animal Society: