By what age will both fontanels have hardened into bone?

The correct answer is two years old because this timeframe aligns with the typical developmental milestones regarding fontanels, which are the soft spots on a baby's skull. At birth, a baby's cranial bones are not fully fused, allowing for flexibility during childbirth and adequate brain growth during infancy. The anterior fontanel, which is the larger soft spot on the top of the head, generally begins to close and harden around 18 to 24 months of age. The posterior fontanel, located at the back of the head, typically closes even earlier, around 2 to 3 months after birth.

By the age of two, both the anterior and posterior fontanels have usually fully fused, resulting in a solid bony skull structure. This process is crucial for protecting the developing brain as the child grows. Understanding this timeline helps in assessing normal developmental stages in young children.