Chronological age is the most obvious marker of age. However, chronological age is not necessarily the best indicator of biological age. An 80 year-old may be physiologically better off than a 65-year-old. The aging process can catch up to different people at different times.

Therefore, scientists are looking for other predictors of life span. While there are several candidates for such "biomarkers of aging," none have so far been proven to be a true measure of the underlying aging process. A true biomarker of aging must meet certain criteria in order to be both accurate and useful.

1. It must predict the rate of aging. In other words, it would tell exactly where a person is in their total life span. It must be a better predictor of life span than chronological age.

2. It must monitor a basic process that underlies the aging process, not the effects of disease.

3. It must be able to be tested repeatedly without harming the person. For example, a blood test or an imaging technique.

4. It must be something that works in humans and in laboratory animals, such as mice. This is so that it can be tested in lab animals before being validated in humans.

Simply put, biomarkers need to be simple and inexpensive to use. They should cause little or no pain and stress. And they must measure aging accurately.