How old do kittens have to be before they can be fixed?

they generally have to be 6 months old to have them desexed, however you can buy kittens that have already been done at the age of 6 to 8 weeks old. Being done at that young age is no guarantee that they can not breed when they get older as they are not fully developed. I know this because I bought a kitten that had been done at a young age and he wants to go out and do all those male cat things, I also have a cat that got done at 6 months old and he has no urges to go out and do those things. My advice to you, is to wait until the kitten is 6 months old.

A vet told me 6-8 weeks old
6 months
Many vets are still recommending 6 months, but the age has been drawn back by many after significant research proved there aren't any permanent damages when doing so at a younger age. Besides that, many female cats can go into heat before 6 months of age, and it's best to get a cat neutered/spayed before they reach any sexual maturation to help with that whole spraying thing.

Some places neuter and spay as early as 2 months..but a general guideline is 4-6 months. I'd do it sooner rather than later.
6 weeks I think, the earlier the better coz they can get pregnant/ get others pregnant from a very young age
my vet flips out when places like the humane society or spca have them fixed so early.. my kitten was fixed when i got her at 11 weeks and he said its not good for them at all. call a vet and ask them!
About 5 - 6 months, no sooner. To early can be bad. They shouldn't be in heat any sooner than 7 months anyway, so anytime before then is good. Too soon may be bad for the animal, you want them to go through puberty first.
kittens have to be fixed before 6 mths 99% sure
Hi's an article written by a vet for the Winn Foundation that indicates early spay/neuter can be done as soon as 6-14 weeks and no later than 5-7 months and explains why:

Early Spay/Neuter in the Cat
by Susan Little DVM
While it may seem that interest in early spay/neuter is a recent phenomenon, it has not only been talked about, but it has been practiced for over 25 years in North America. Early age altering refers to spays and neuters done between the age of 6 and 14 weeks. Altering pets between 5 and 7 months of age was established by tradition rather than for any specific medical reason. Years ago, when safe pediatric anesthetic techniques were not available, waiting until a patient was older increased the safety of surgery. But we no longer need to delay altering for this reason.

People working to decrease the problem of surplus dogs and cats in the United States pioneered the idea of early altering. While surgical sterilization remains the most effective means of population control, delaying the surgery long enough for sexual maturity to occur defeats the purpose. Animal shelters advocate mandatory altering, but many adopted animals either are never altered or have at least one litter first.

Over the years, the safety of early altering has been questioned, mainly by veterinarians who may be unfamiliar with the surgical and anesthetic techniques required for pediatric patients. As well, concerns that early altering could increase the incidence of feline lower urinary tract disease, could affect skeletal development, and affect behavior have been voiced. These concerns have largely been laid to rest by many studies, and early altering is becoming more widespread and available. A study recently published by researchers at the University of Florida found no significant differences in the physical and behavioral characteristics of cats altered at 7 weeks of age compared to those altered at 7 months of age.

Very important work has been done by Drs. Michael Aronsohn and Alicia Faggella at the Massachusetts SPCA on the anesthetic and surgical techniques for early altering of dogs and cats. In 1993, two papers were published outlining their work on the early altering of hundreds of kittens between the age of 6 and 14 weeks. They evaluated several anesthetic protocols and made recommendations for safe handling and anesthesia in patients of this age. Some small changes to surgical technique are necessary for patients in this age group. As well, these young patients must be handled a bit differently both before, during, and after surgery. The changes in surgical protocol are simple and easy to carry out, and the experience of these veterinarians with early altering is overwhelmingly positive.

As cat breeders, we must do our part to curtail the serious issue of surplus animals. Many of us work in breed rescue programs and give our time and expertise to shelters. We can ensure that our own kittens not destined for breeding programs will never reproduce by practicing early altering. Early altering is a safe and effective means of ensuring we do not unwittingly add to the burden of unwanted pets.

Another article study about early spay/neuter vs. traditional spay/neuter time frame:
Usually 6-8 weeks, at least back in Canada. Now I live in Thailand, and vets don't like doing it before 6 months, which isn't good, as a female can sometimes go into her first heat by that age.
The vet told me six months. When we lived in Phoenix we adopted a kitten from the humane society and they dont let any animal leave their shelter until it is neutered. He was 8 weeks old and is now 8 years old and is fine

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