The biological age of a individual human being is often clinically different from the characteristics expected of an individual of that chronological age. Indeed whole organ or organ systems in an individual might have differing biological ages. These biological ages differences might be (with some extreme examples indicated)
- Smoking The increasing dissociation between facial appearance (skin winkling) and biological facial appearance as expected for the same chronological age is an excellent example of how chronic environmental exposure to an agent that is a mutagen and causes increased protein cross linking.  
Possible mechanisms include:
- Chemical alteration of the DNA, eg methylation with some selection where this is allowed
- eg if methylation was allowed on miRNA DNA codes but not on pre-mRNA DNA codes
- Counting DNA repairs.
- DNA repair in man is not always perfect, indeed this is a cause of malignant transformation due information introduced at the site of the repair rarely
- Telomere shortening
- Chemical alterations in protein
- Mitochondrial DNA damage
- ↑ Model D. Smoker's face: an underrated clinical sign? Br Med J 1985;291(6511):1760-2
- ↑ Boyd AS, Stasko T, King LE, Cameron GS, Pearse AD, Gaskell SA. Cigarette smoking-associated elastotic changes in the skin. Journal of the American Academy of Dermatology 1999;41:23-6.
- ↑ Koh JS, Kang H, Choi SW, Kim HO. Cigarette smoking associated with premature facial wrinkling: image analysis of facial skin replicas. International journal of dermatology 2002;41:21-7.
- ↑ Raitio A, Kontinen J, Rasi M, Bloigu R, RÃ¶ning J, Oikarinen A. Comparison of clinical and computerized image analyses in the assessment of skin ageing in smokers and non-smokers. Acta dermato-venereologica 2004;84:422-7.
- ↑ Leung WC, Harvey I. Is skin ageing in the elderly caused by sun exposure or smoking? The British journal of dermatology 2002;147:1187-91.
- ↑ Preston CR, Flores C, Engels WR. Age-dependent usage of double-strand-break repair pathways. Current biology : CB 2006;16:2009-15. (Direct link – subscription may be required.)
- ↑ Aronson D. Cross-linking of glycated collagen in the pathogenesis of arterial and myocardial stiffening of aging and diabetes. Journal of hypertension 2003;21:3-12. (Direct link – subscription may be required.)
- ↑ Wiesner RJ, Zsurka G, Kunz WS. Mitochondrial DNA damage and the aging process: facts and imaginations. Free radical research 2006;40:1284-94. (Direct link – subscription may be required.)