How likely is it for a 13 year old to have gum disease?

Bacteria: can cause damage in the mouth, young or old - if there's an environment for it to to thrive. If the acid it produced can cause decalcification and decay on the enamel, which is the hardest structure in the body, the more it won't spare the soft tissue.

Very likely: at any age. The bacteria, along with mucus and other particles, constantly form a sticky, colorless toxic “plaque” on teeth causing the gums to become inflamed and to easily bleed. This is a serious problem, which should be checked by a dentist. The good news is that this condition is curable. Call your dentist today!