How do you tell when to change the height of crib mattresses?

Developmentally: If your baby can sit up on his/her own and is showing signs of trying to pull up to a standing position, it's time to drop the crib mattress as low as it will go. Don't wait until your baby is already able to stand up; some six-month-olds have fallen out of their cribs! once you lower the mattress, it's time to take out accessories like bumpers as well.

Mobility: Once your child begins to get on his/her knees, or pulls to stand, the crib mattress should be lowered all the way to prevent climbing and falling.