Although most of the cavities that dentists find in children and young adults are either pit and fissure decay(located in the occlusal surfaces of posterior teeth) or interproximal decay, located immediately below the contact area, another common place to find decay is along the gum line of teeth.
Gum line caries may be caused by a number of factors, including excessive consumption of sugary foods, sodas, bulimia, acid reflux, lemon sucking, dry mouth and general poor brushing technique. Caries usually starts with acid dissolution of tooth structure and invasion of the tooth structure by bacteria. Normally smooth surface enamel is fairly resistant to caries, especially when it is cleaned well by brushing . Sometimes even 'good brushers' can have areas that they find hard to keep clean. It is not terribly unusual to find decalcifications on the buccals of upper second and third molars, since these tooth surfaces are sometimes hard to get to with a toothbrush.
Often children both, young and adolescent, don't spend the time needed to brush their teeth systematically. Many just distribute their tooth paste around their mouths in a rather random fashion and assume that the toothpaste will 'magically' clean their teeth. This is not the case, since toothpaste doesn't actually do much cleaning. Instead the tooth brush bristles are doing most of the cleaning and they should contact all accessible tooth surfaces.
Older patients( over seventy) often present with root caries. These carious lesions tend to be located on roots of teeth that are exposed as a result of gingival recession. Cementum normally covers root surfaces, but often this thin coating get denuded over time and exposed dentin can be more at risk for decay especially, for seniors, who tend to experience the double whammy of both dry mouth and a decline in oral hygiene habits. These carious lesions can be a real challenge for dentists to restore since they can be in difficult to reach areas and restorations can experience recurrent decay afterwards.
Accordingly, to restore root caries, I almost always use high fluoride releasing restorative filling materials, such as glass ionomer formulations(Gc Miraclemix, Gerastore, RelyX) to restore these lesions, since they offer some protection from future recurrent decay.