Physical signs and behaviors are the best ways to determine the age of a kitten. Kittens develop in their own time based on genetics and environmental factors.
For the first week, kittens are helpless, have their eyes and ears closed, and spend nearly all their time sleeping. Two-week-old kittens have their eyes and ears open and can wobble about. Three-week-old kittens begin to eat solid food and can walk steadily. At four weeks, kittens start to play, while five-week-old kittens start showing their personality. By six weeks, kittens are able to groom themselves. At 12 weeks, kittens are fully weaned and independent.