Age at onset (AAO) has frequently been a key indicator in delineating disorder subtypes leading to gene identification. Thus far, differences in AAO have helped to separate genetic from sporadic cases in common illnesses such as breast cancer.1 Differences in AAO may also be used to identify different vulnerability genes, as in Alzheimer disease,2 or different mutations in the same gene, as in Duchenne-Becker muscular dystrophy.3 More recent findings have shown that AAO may also reflect differential expansion of an unstable DNA region at or near the disease locus, as in myotonic dystrophy4 and Huntington disease.
Bellivier F, Golmard J, Henry C, Leboyer M, Schürhoff F. Admixture Analysis of Age at Onset in Bipolar I Affective Disorder. Arch Gen Psychiatry. 2001;58(5):510-512. doi: