A wisdom tooth refers to any of the usual four third molars. Wisdom teeth usually appear between the ages of 16 and 25. Most adults have four wisdom teeth, but it is possible to have fewer, or more. Wisdom teeth commonly affect other teeth as they develop, becoming impacted or erupt in a sideways or "non-linear" fashion. Often when this occurs, it is necessary to extract them. Sometimes these teeth can be a valuable asset to the mouth when healthy and properly aligned, but more often, they are misaligned and require removal.