How do you determine if a supposed longevity therapy is working in humans short of waiting for decades? This is why we need reliable biomarkers for biological rather than chronological age: measures of how much your body's systems have changed and degenerated from youth. From EurekAlert!: researchers have "found that as cells and tissues age, the expression of a key protein, called p16INK4a, dramatically increases in most mammalian organs. ... Because p16INK4a is a tumor suppressor protein, cancer researchers are interested in its role in cellular aging and cancer prevention. Now the team has proven that the same biomarker is present in human blood and is strongly correlated both with chronological age and with certain behaviors such as tobacco use and physical inactivity, which are known to accelerate the aging process. ... We found a very weak correlation between the biomarker and obesity - as measured by body mass index (BMI) - despite other data suggesting that caloric restriction slows aging. The data suggest the possibility that reduced exercise may actually be worse with regard to molecular age than a higher BMI." Bear in mind that there will be no one biomarker that gives an unbiased view of aging: a mammal is a very complex collection of interacting and very different system.