Cats stop growing when they reach adulthood -- usually around their 1st birthday. Their bones stop growing at about 8 months old and they begin sexual maturation, which usually takes another four months. Growth speeds vary by breed, but all cats have special nutritional requirements at all stages of life.
Skin and Bones
Despite the preponderance of kitten videos online, a newborn cat won't stay small for very long. Kittens have a steep growth curve, particularly during their first six to nine months. During this time, your kitten's bones grow and the size of his body increases exponentially.
In the following four months, a cat reaches sexual maturity, which causes more growth, although it's not as dramatic as what proceeded it. Your cat may fill out or fatten up during this period.
These processes vary by breed -- details about adult cat size are available for all 42 recognized breeds through the Cat Fanciers' Association -- but are usually most pronounced in a cat's first year of life.
Your cat may stop growing or grow more slowly if his nutritional requirements aren't met. Too little magnesium, sodium, potassium, iron and zinc can retard growth, while too much calcium causes denser, slower-growing bones.
Many veterinarians recommend high-quality commercial kitten food. In the U.S., any pet food marketed for a particular life stage must meet standards set by the the Association of American Feed Control Officials. The same is true for the "complete and balanced" labels. Approved foods will likely help your cat keep growing.
Growing cats have higher caloric and nutritional requirements, but they also have smaller stomachs. Free feeding is a convenient way to foster frequent meals, especially for underweight or slow-growing kittens, but it can also lead to obesity. Neutered cats may be especially prone to this, although data sometimes conflicts.
If you're feeding a kitten meals, measure a consistent amount -- 3/4 cup of dry food is a general guideline, but by no means the rule -- and offer food three or four times a day. When he turns 6-months-old, switch to twice daily feedings.
When your cat turns 1, he's pretty much done growing. For the sake of comparison, he's like a 21-year-old human. After a cat turns 1, every year is roughly equivalent to four human years, veterinarian Dr. Arnold Plotnick says. As such, a 20-year-old cat is like a 101-year-old person, although cat age and health don't necessarily follow a one-to-one comparison to humans.
During your cat's second year of life, he may continue filling out, but his bones are set and his nutritional requirements are stabilizing.