Final answer:
The sutures in the skull completely close around the age of 2 to 3 years old.
Step-by-step explanation:
In newborns and infants, the areas of connective tissue between the bones of the skull are much wider, and they are called fontanelles. As the skull bones enlarge, the fontanelles gradually decrease in width and are replaced by narrow fibrous joints called sutures. The process of sutures closing and fusing into bone is called synostosis. The timing of complete closure of the sutures varies, but typically occurs around the age of 2 to 3 years old. The sagittal, coronal, and lambdoid sutures of the skull will begin to ossify and fuse, causing the suture line to gradually disappear.