My niece had a problem with the vertebrae just under her skull, but her doctor didn't detect it until she was five years old. Now my sister has to be careful and not allow my niece to take part in certain activities, like diving. Shouldn't this have been discovered sooner?

Upper back injuries in children often are undetected for quite a while because it takes the spine until the teens to be fully hardened. Certain parts of the spine will be finished before other parts.

There are some disorders or anomalies in the upper spine and neck that might not be obvious until a child is four or five years old because that's when the vertebrae finish fusing together. Unless a child has a specific disorder, like Down's syndrome, that commonly has anomalies like this, or unless a child has an injury or problems that make doctors examine that part of the spine, it's entirely possible that the problem go undetected for quite a while.