In actuality, crib bumper pads should never be used in the cribs of children under a year old, in addition, they can cause problems in children up to 2 years of age. They have been connected to cases of SIDS when unable to pull themselves free. However, past the age of 1.5 years, this risk is greatly decreased.
However, this only applies to traditional bumpers. There is a new form of bumpers, the mesh breathable crib bumper pads, that are safe to use in cribs after a child is just a few months old.