Age of onset

Diseases are often categorized by their age of onset as congenital, infantile, juvenile, or adult. Missed or delayed diagnosis often occurs if a disease that is typically diagnosed in juveniles (such as asthma) is present in adults, and vice versa (such as arthritis). Depending on the disease, ages of onset may impact features such as phenotype, as is the case in Parkinson's and Huntington's diseases. For example, the phenotype for juvenile Huntington's disease clearly differs from adult-onset Huntington's disease and late-onset Parkinson's exhibits more severe motor and non-motor phenotypes.

Causes

Germ-line mutations are often at least in part the cause of disease onset at an earlier age. Though many germ-line mutations are deleterious, the genetic lens through which they may be viewed may provide insights to treatment, possibly through genetic counseling.

In some cases, the age of onset may be the result of mutation accumulation. If this is the case, it could be helpful to consider ages of onset as a product of the hypotheses depicted in theories of aging. Even some mental health disorders, whose ages of onset have been found to be harder to define than physical illnesses may have a mutated component. The symptoms of standard mental disorders often start off non-specific. Pathological changes pertaining to disorders often become more detailed and less fickle before they can be defined in the American Psychiatric Association's DSM. The brain is a dynamic and complex system, it is constantly re-wiring itself and a major concern is what happens to the brain in earlier life that mirrors what occurs later in its psycho-pathological state. The typical onset of many mental disorders in late adolescence may reflect the critical development that happens at this time.