Until the age of 1 year, a cat is considered to be a kitten. Typically, for most breeds, full growth is reached by 9 - 12 months of age. After 12 months some cats may still continue to increase in weight and stature but most breeds should have reached their full adult potential by 18 months of age at the latest.
Generally, most cat breeds are considered to be an adult between the ages of 1-8 years of age. Your cat will be in the prime of his/her life during this period. Usually most healthy adult cats will have normal kidney, liver and heart function and most are still free of other debilitating conditions such as osteoarthritis but their nutritional requirements will be different to that of a kitten or a senior pet.
Generally, your cat is medically considered a senior from the age of 8 years and above. It may appear that there is little difference outwardly between an adult and a senior cat, but from a medical point of view, your cat is aging significantly inside” from the age of 8 onwards.