Permanent or Baby? Typically the baby tooth is lost at about 7 years old and the permanent version hopefully will last forever. If this is a baby tooth that was not lost until age 13 it is possible the permanent tooth never formed or is impacted (stuck) under the gum. The situation needs to be verified by a Dentist and appropriate action undertaken depending on the cause.
Possible. Without an examination or xrays its very difficult to give definitive answers. It may have been a retained primary tooth. The permanent may be missing or impacted below the gum. It would be unusual to lose a permanent tooth so young without trauma or serious periodontal disease.
Unusual. All of the baby incisors should have fallen out by Age 8, unless the underlying permanent tooth is congenitally missing or impacted. Exceptionally rare for a permanent lower incisor to be lost so young, especially in the absence of trauma or gum disease. Please have your General Dentist evaluate your occlusion. By 28 permanent teeth should have erupted. Orthodontic consultation may be in order.