Kittens typically become adults around their first birthday, though some larger breeds may take up to 18 months to reach full physical maturity. After that point, they’re generally considered full-grown cats rather than kittens. Growth and development occur in stages, with noticeable changes in body size, coat development, and behavior often continuing through the first year and into the second for larger breeds. Key milestones

• Four to six months: sexual maturity can begin in many cats, and they may enter puberty if not spayed or neutered. This doesn't mean they’re fully grown, just that hormones are active.

• Around 12 months: most cats are considered adults and are fully grown in height for many breeds. Some breeds (e.g., Maine Coons) may continue growing in size and weight beyond one year.

• 18 months to 2 years: larger breeds may achieve final body size and maturity more gradually, culminating in physical and behavioral adulthood.

What influences timing

• Breed size: small to medium breeds often reach full physical maturity closer to 12 months, while large and giant breeds can take 18–24 months.

• Individual variation: genetics, nutrition, and overall health can shift the timing of growth and maturity. Regular veterinary checkups help assess an individual cat’s development and optimal weight.

Behavioral aspects

• Kittens are often very active, curious, and energetic for several months beyond infancy. Most cats settle into more adult-like behavior as they pass the first year, though some younger cats remain lively longer.

If you want to pinpoint your specific cat’s maturity, your vet can assess growth plates (in growing kittens) and overall development to confirm when they’re best considered fully grown for health and care planning.