While there are plenty of myths and wive's tales surrounding when it is best to get your pet fixed, it is agreed upon by veterinary professionals that cats should be fixed prior to 4 months of age. In shelters, some cats are fixed as small as 8 weeks with no negative side effects! And it is absolutely 100% false that you should let your cat have at least one litter of kittens before having her spayed. A cat can have kittens at as young as 4 months old, and often has upwards of 4 kittens in a litter. Then, 4 months later, all of those kittens can have babies! AAHHHH! That's just too many kittens to find homes for, especially with shelters overflowing as it is. Instead, it is recommended that you keep your growing kitten away from unaltered cats of the opposite sex, and have them fixed somewhere around the 4 month mark. As far as dogs, while some smaller dog breeds can be fixed at the 8 week mark, many vets advise to wait longer for larger dogs. When your puppy is getting their first shots (usually around 8 weeks) talk to your vet about what age they recommend doing the procedure. It's a fine line, as performing it too early can stunt their growth, while performing too late can put them at an increased risk of different types of cancers.
There are so many reasons to have your cat fixed. First of all, we have already touched on the exponential growth of unaltered cats. If you are providing adequate veterinary care for your momma cat and all her kittens, it will run you thousands of dollars. Between checkups, vaccines, deworming, flea and tick prevention, the spay and neuters of the kittens, and any unexpected vet bills (because with kittens, it usually happens) it will add up very quickly.
The next thing you want to consider, is the undesirable behaviours that come with intact male and female cats. Female cats can go into heat quite often in the warmer months, while males are more prone to fighting, spraying, and will cry to get outside if they hear/smell a female cat in heat.
Additionally, desexed animals, on average, live longer! In female cats and dogs, it prevents the risk of many types of cancer (including breast, and ovarian), birth complications, and uterine infections. In male cats and dogs, you eliminate the risk of testicular cancer, lower the risk of prostate cancer significantly, and lower the likelihood that your dog could be injured in a fight with another dog.
A common concern is that spaying/neutering your pet will cause them to become overweight, but this is not exactly true. While there is some correlation found, it's not as cut and dry as that. Animals are more at risk for becoming overweight as they age, and most animals being fixed are coming out of kittenhood/puppyhood and growing into adulthood! The lowered amount of hormones after desexing causes a decreased metabolic rate, which combined with an animal aging out of their juvenile stage and owners still feeding the amount of food a younger animal would need for growth, can lead to obesity. But spayed and neutered cats and dogs can still maintain healthy physique! As long as they are receiving a good diet and proper exercise, there is nothing to worry about. Some vets advice feeding your neutered cat or dog about 80% of what you fed them pre-op, but we find it best to calculate how many calories your pet should receive daily, and feed based off of that. If your pet starts to develop fat pads that inhibit you easily feeling their ribs or lower vertebrate with mild pressure, you will want to reduce their calories.