Researchers are making better progress of late in finding ways to use changes in telomere length that occur with aging as a marker for biological age and life expectancy - though it remains an open question as to whether telomere shortening is a cause of aging versus a secondary consequence of causes of aging. You might look at work in mice published earlier this month, for example. Or moving to birds, a few years back researchers noted that pace of telomere shortening over time correlated with lifespan differences between species. Here researchers consider telomere length in a species of warbler:
[Researchers] studied the length of chromosome caps - known as telomeres - in a 320-strong wild population of Seychelles Warblers on a small isolated island. ... Over time these telomeres get broken down and become shorter. When they reach a critical short length they cause the cells they are in to stop functioning. This mechanism has evolved to prevent cells replicating out of control - becoming cancerous. However the flip side is that as these zombie cells build up in our organs it leads to their degeneration - aging - and consequently to health issues and eventually death. Telomeres help safeguard us from cancer but result in our aging.
We wanted to understand what happens over an entire lifetime, so the Seychelles Warbler is an ideal research subject. They are naturally confined to an isolated tropical island, without any predators, so we can follow individuals throughout their lives, right through to old age. We investigated whether, at any given age, their telomere lengths could predict imminent death. We found that short and rapidly shortening telomeres were a good indication that the bird would die within a year.
We also found that individuals with longer telomeres had longer life spans overall. It used to be thought that telomere shortening occurred at a constant rate in individuals, and that telomere length could act as an internal clock to measure the chronological age of organisms in the wild. However while telomeres do shorten with chronological age, the rate at which this happens differs between individuals of the same age. This is because individuals experience different amounts of biological stress due to the challenges and exertions they face in life. Telomere length can be used as a measure of the amount of damage an individual has accumulated over its life. We saw that telomere length is a better indicator of life expectancy than chronological age - so by measuring telomere length we have a way of estimating the biological age of an individual - how much of its life it has used up.