Generally, cats reach adulthood around 8 months to 1 year old. "Adulthood" means reaching basic sexual maturity, so once a cat reaches sexual maturity, it can be considered "adult". 
When a cat reaches 12 months old, it is considered an adult cat. At this time, all its organs are fully developed, and it can mate normally during its heat cycle. Its food can also be changed to adult cat food.
After a cat reaches adulthood, its bones stop growing and its body size becomes fixed. At this time, it is necessary to control its food intake, otherwise it is easy for it to become obese.
