Telomeres are the protective caps of repeated DNA sequences stuck onto the end of chromosomes, cut short with each cell division, but maintained by an enzyme called telomerase whose job, amongst others, is to extend telomeres by adding extra repeats. As you can imagine, this lays the groundwork for complex feedback loops, influenced by many genes, and different in different species and cell types. Telomere biology is associated with aging, and telomeres tend to shorten in some species and some tissues with both advancing age and ill health - but it's still an open field for the development of a full explanation as to exactly how and why that is the case. Of great interest is whether the erosion of telomeres is one of the few primary causes of aging, or whether it is only a secondary consequence - for example, do telomeres erode because of mitochondrial damage?
At the moment, the best possible outcome would be that telomeres turn out to be a primary cause of aging and the various groups working on telomerase-based therapies wind up producing a useful tool for the rejuvenation toolkit. A more plausible outcome would be that telomeres are a useful biomarker of health and remaining life expectancy but don't actually require any specific medical intervention - other forms of rejuvenation biotechnology will address the primary causes of aging, and telomeres will lengthen as a result. But we shall see where it all ends up.
There has been a fair amount of back and forth as to just how well telomere length can serve as a biomarker: lots of different outcomes in different studies. To some degree this was expected due to the differing behavior of telomerase in different tissues, but even so there is much to debate given the results to date. Here, a study in birds provides further food for thought by showing that telomere length varies in usefulness as a measure at different periods in life:
The birds with the longest telomeres - the protective caps at the ends of chromosomes - live the longest, according to a new study. "It is the first time this has been shown for any species," ... The scientists measured telomere length in red blood cells of 99 captive zebra finches (Taeniopygia guttata). The birds resemble long-lived animals in that there is little restoration of telomeres in body cells as they age. The first measurement was taken at 25 days; the researchers then followed the birds over their natural life span, ranging from less than a year to nearly 9 years, and measured telomeres again at various time points. They found a highly significant correlation between telomere length at 25 days and life span; birds with longer telomeres lived longer. Length measured at 1 year also predicted life span, but the relationship was weaker, whereas at later time points (after 3, 4, 6, and 7 years) there was no correlation ... This might explain why previous results in humans and animals have not been consistent. "So far studies just looked at individuals that were already quite old," Monaghan says. "But if you look at telomeres in old age, then those individuals with the shortest telomeres will have already died."
And birds, by the way, are not necessarily the best model for thinking about telomeres in mammals - they are quite different, possibly as a result of the metabolic demands of flight given that bats exhibit many of the same tropes. It has to be said that the general theme in biology is that the situation inside a living organism is always more complex than was thought a few years ago, and definitely more complex than we'd like. The foundations of medical technology would be easier if we were simpler.