Telomeres are the repeated DNA sequences found at the ends of chromosomes. A little of that length is lost with each cell division, and this serves as a part of the mechanism that limits the number of times a somatic cell can divide. Stem cells employ telomerase to maintain long telomeres through the asymmetric divisions needed to supply tissues with new daughter somatic cells equipped with long telomeres. This split of responsibilities between many restricted cells and a few privileged cells is the primary strategy by which multicellular organisms keep the risk of cancer low enough for evolutionary success.

Given this arrangement, average telomere length in any given tissue is a blurred measure of how fast cells divide and how frequently new cells are delivered by the supporting stem cell population. Over large populations of people, shorter telomere length tends to correlate with greater age, most likely because stem cell activity declines with age. Unfortunately, it is the case that telomere length as presently measured - in leukocytes from a blood sample - is quite dynamic in response to day to day environmental circumstance, and is thus only poorly correlated to aging for any given individual. Telomere measurement services are readily available, but there really isn't all that much that can be deduced from the result. It isn't actionable. If measured again next week or next month, or with a passing infection versus without, then the number will likely be significantly different.

Further, for every study population in which the correlation with aging is affirmed, there is another in which the telomere length data stubbornly refuses to do the expected thing. The study here produces both of these outcomes, confirming the correlation in younger people, but also finding that the relationship falters for individuals older than 80 years of age. All in all telomere length just isn't a very useful measure of aging. It is not robust enough, and its individual variability means that the numbers are next to useless when it comes to guiding medical decisions.

Telomere Length and All-Cause Mortality: A Meta-analysis