Your sister is wrong and uninformed. Her opinions are not based on fact.
Cats spayed/neutered at 7 weeks show no differences in health, growth, or behavior than cats spayed at 7 months. In fact, they are generally larger, not scrawnier (makes sense if you think about it - that energy goes into growth, not into "sexual maturity." )
Both of these groups of cats tend to weigh a little more than cats spayed at over 1 year old, but if you read the publication you will see weight is not an issue because of the spaying/neutering and they were not "fat" or lazy cats. I don't know anyone, anywhere or any vet that recommends cats be spayed at anything older than 6 months.
The truth is that early age spay neutering (as young as 7 weeks) is safe.
Basically, the short and long term effects of early spay/neutering were compared between three groups. The only real difference in the early spay/neuter group is that they healed up faster.
Here were the groups used in the study:
Group 1, those spayed/neutered at 7 weeks
Group 2, those spayed/neutered at 7 months
Group 3, those spayed/neutered after sexual maturity at 12 months
|Developmental and Behavioral Effects of Prepubertal Gonadectomy. Mark S. Bloomberg, DVM, MS; W.P. Stubbs, DVM; D.F. Senior, BVSc; Thomas J. Lane, BS, DVM; University of Florida at Gainesville. Funded by the Winn Feline Foundation, February 1991. Continuation funded February 1992.
There was generally no difference in food consumption between the three groups other than the differences between males and females, which were consistent in all groups. There was no difference observed in the growth rates in all three groups, although the males grew faster in all groups. Increased long bone length was observed in both males and females in Groups 1 and 2. This appeared to be due to the fact that physeal closing (closure of the bone growth plate) was delayed in Groups 1 & 2. This explains why cats neutered and spayed as kittens are frequently larger (longer and taller) than unaltered cats or cats altered later in life. This seems to be particularly true for males.
In terms of behavior, after 7 months, the cats in Group 3 were noticeably less affectionate and more aggressive prior to altering than the cats in Groups 1 and 2. Contrary to popular opinion, neutered animals were as active as their unaltered age mates.
Observations of urinary tract development showed no differences between the three groups other than the differences related to sex and these were consistent across all groups. The investigators measured the diameter of the urethra in the male kittens only and found no differences between the groups. Concerns have been raised that early neutering would result in smaller diameters in the urinary tract, resulting in an increased incidence of cystitis and related problems. This does not appear to be the case.... Concerns that development of the urinary tract might be arrested or impaired by early spaying and neutering proved unsupported.
The results of this study so far indicate that the differences between cats neutered at 7 weeks and 7 months are insignificant.
Most vets go by weight rather than "week" age, and 2 pounds is usually the requirement.