If the "soft spot" hasn't closed at 18 months, could there be a problem?
The typical anterior fontanelle (most commonly known to parents as the "soft spot") is generally between 3 to 6 centimeters diameter during the first 6 months of life, and gradually gets smaller thereafter—closing some time between the ages of 10 and 25 months old. Any babies who are born prematurely, however, may have their anterior fontanelles close even later. Given these normal ranges, having an open anterior fontanelle at 18 months is not automatically of concern, but rather something worth discussing with your pediatrician.
For a little background, the anterior fontanelle is simply an area on top of a baby's head where the various bony plates that make up the skull have not yet come together. The fact that a newborn's skull isn't one solid bone at birth, but rather comprised of several bony plates is not just useful, but necessary, as it allows a newborn's head to adjust enough in size to pass through the birth canal and then allow the skull to expand as babies' brains grow considerably throughout the first year.
While having an open anterior fontanelle at 18 months is still within the range of normal, it's nevertheless worth getting checked and having your pediatrician follow along to make sure it is closing normally, as there are several known reasons for delayed closure. The most common causes can include simply a normal variation, but can also be a sign of an underactive thyroid (congenital hypothyroidism), increased pressure inside the skull (increased intracranial pressure), Down syndrome, a condition called primary megalencephaly, rickets, or several other less common syndromes that are generally associated with other physical abnormalities aside from just an open anterior fontanelle.
The best thing to do whenever there is a question about a baby's fontanelle is to simply have your baby's pediatrician examine your baby and determine whether the open fontanelle warrants further evaluation or if it's simply closing gradually as expected.