Biomarkers of aging are physical properties in the human body which indicate that the body is aging. It is indicators of the normal phenomena of growing old. They are not, however, simply things which change with age. In order to be called a biomarker, a factor has to satisfy a number of criteria. The best markers will be the ones which are not susceptible to influence from the outside environment. For example, in the US cholesterol levels increase with age, but this is due to the nature of the American diet and is not characteristic for other parts of the world. Thus, a true biomarker would satisfy the following criteria:
A. The marker must predict the rate of aging and be a better predictor of life span than chronological age.
B. It must be able to be tested on a regular basis
C. It must work both for humans and other species, such as laboratory animals
D. There is support from human clinical assessment and complementary research studies.
E. The studies are based on a significant representative sample.
F. The result is a clear association with aging.
G. A relatively narrow standard deviation is present.
So far, around 24 factors have met the criteria and can be considered biomarkers. They may be indicated especially for males or for females, and figures may vary between the sexes. Here is their list:

1. 17-ketosteroid/ 17-hydroxycortiosteroid ratio (male) 13. Handgrip strength
2. Ascorbic acid 14. Hemoglobin A1C
3. Basal Metabolic Rate 15. Lung capacity- FEV1
4. Blood pressure- pulse 16. Lung capacity- FVC
5. Blood pressure- systolic 17. Maximum oxygen update (male)
6. Body Mass Index (female) 18. Near vision
7. Caries index 19. Noradrenaline- plasma (male)
8. Creatinine clearance 20. Peridontal index
9. DHEA-S 21. PSA total (male)
10. Fibrinogen 22. Skin elasticity
11. Hair baldness (male) 23. Testosterone free (male)
12. Hair grayness 24. Zinc- serum

In addition, there are also a number of other factors which may be considered partially biomarkers of aging. The main problem with these is that their reliability has not been confirmed through a sufficient amount of clinical and experimental data. These include body flexibility, blood urea nitrogen, LDL cholesterol, melatonin levels, static balance, serotonin levels and many others. They are to a certain degree indicative of a person’s biological age, but should not be confused with other general health factors, which do not have a clear association with age.
Biomarkers of aging could be divided in three major categories. There are the ones which determine the biological age, e.g. skin elasticity and visual accommodation. There are markers which predict the remaining life expectancy; they include DHEA-S, hand grip strength, etc. Finally, there are factors which determine disease susceptibility, such as systolic blood pressure and glucose-tolerance tests. All of the biomarker tests can be classified either as laboratory tests (e.g. blood and urine tests) or as physical tests undertaken in a clinic.

from: J Steroid Biochem Mol Biol. 2003 Jun;85(2-5):329-35.