As long as the child can still fit in the crib and will stay in the crib at night then they can sleep in the crib until they can no longer fit or be comfortable sleeping in the crib which is around 4 years old.
But most kids will be ready to move out of the crib before they actually outgrow the crib in size and will start climbing out of the crib as soon as they can start standing up and walking.
By 1 year and a half the child is usually ready to move out of the crib and into a toddler bed but stay in the crib until 3 or 4 years of age which is okay.
Some people let their kid stay in the crib until past 3 years old and then move them directly to a regular sized bed which is okay.
The longer the child stays in the crib the safer they are from falling out of bed or wandering around the house at night unless they climb out of the crib.