The sutures of the skull fuse around the brain at around age 2 years. When a baby has craniosynostosis, one or more of these sutures hardens too early and closes before the baby reaches age 2.

At what age does the skull harden?

When babies are born their skulls are soft, which helps them pass through the birth canal. It can take 9-18 months before a baby's skull is fully formed.

At what age does the cranium turn to solid bone?

Normally, these sutures stay open until babies are about 2 years old and then close into solid bone. Keeping the bones flexible gives the baby's brain room to grow. When the joints close too early, the brain pushes against the skull as it continues to grow. This gives the baby's head a misshapen look.