The AAP recommends moving your child to a toddler bed when he is 35 inches (89 cm) tall, or when the height of the side rail is less than three-quarters of his height.
Lowering the mattress is fairly easy to do and only takes a few minutes. Simply remove the mattress from the crib and then re-install the frame at the lower setting.
You can move your child to a toddler bed as early as 18 months, but some kids may not be ready until they’re 2 or 3 years old. It’s important to listen to your child and follow their cues.