Kittens generally refer to the period from birth to about one year old. Specifically, kittens' ages can be divided as follows:

From birth to about 4 weeks: This stage is called the neonatal period or infancy, during which kittens mainly rely on their mother's care and nursing.
Around 4 to 8 weeks: This stage is known as infancy. Kittens begin to be weaned and gradually start eating solid food. They will begin to explore their surroundings and learn basic social skills.
Around 8 to 12 weeks: This stage is known as adolescence. Kittens become more active and curious, and begin to learn independent behavior and feline social rules.
Approximately 3 to 6 months: This stage is known as the growth period, during which kittens continue to grow rapidly and gradually become fully weaned. They may become more active and react more strongly to external stimuli.
Around 6 months to 1 year old: This stage is called adolescence, when kittens further develop into adult cats, their bodies gradually mature, and sexual maturity may also begin to appear.
Please note that this is only a general timeframe; each cat's growth rate and timeframe may vary. Different breeds and individuals may have different growth and development processes. When raising a cat, appropriate care and feeding should be provided based on the individual cat's condition and the advice of a veterinarian.