There are two schools of thought in regards to this question.
In most cases, kittens are considered kittens until their first birthday and should eat kitten food for that time. Exceptions do exist for larger breeds, like Maine Coons, who might not mature until a year and a half or even their second birthday.
Another school of thought suggests that kittens should start eating adult food when they hit 80-90 percent of their adult weight, and this happens around nine months of age.
When the time does come to switch a diet from kitten to adult, do it over a week to as many as 10 days. Start small and gradually increase the amount of adult food daily.
As always, your bet is the best person to make the determination of when and how to switch.