In developing the Birch Kids Natural Mattress, we followed industry guidance around the support required for young sleepers as they grow. The firmer side of our mattress provides a bit more support for kids who are transitioning from a crib mattress. As kids grow and develop the softer side offers a bit more plush comfort. The age ranges provided for the Kids Natural Mattress are designed as suggestions. We recommend assessing your child's size and sleep habits to determine which side of the mattress they should use.