Telomeres are lengths of repeated short DNA sequences that cap the ends of chromosomes. The process of cell division shortens telomeres, and they form a part of the cell division counter that gives most somatic cells an expiration date after which they cease dividing. Telomeres are lengthened by the activity of the enzyme telomerase, which is more active in some cells than others - such as in stem cells, which need long telomeres so that they can divide to produce fresh new long-telomere somatic cells to keep tissues healthy and well-maintained.
When researchers measure telomere length in some specific group of cells, they are taking a snapshot of the blurred results of numerous processes in many cells, such as telomerase activity and pace of cell replacement by stem cells, that are themselves affected by near every aspect of health and environment. The most common measure of average telomere length in white blood cells is very dynamic, for example, rising and falling based on day to day health, even though over a lifetime it tends to decrease. But simple measures of average length tend not to capture these effects well, and the precise details of how a telomere length snapshot is taken make the difference between a result that is meaningless, and has no correlation to health, and a result that does tend to correlate with age, health, and future life expectancy.
So we have results like this one in which researchers run a rigorous study and find no correlation whatsoever between telomere length and mortality risk. You can compare that with animal studies that used a variety of techniques for telomere measurement and number crunching, such as proportion of very short telomeres versus average length, that do show good correlations with life expectancy and health. The sum of all this seems to me that rushing out to have your telomere length measured by one of the new services started in recent years is premature:
Human chromosomes are capped by protective ends called telomeres. These ends are shortened during renewal of tissue and eventually become critically short, causing cells to become senescent or die. It is widely believed that lifestyle features such as smoking, obesity, physical inactivity, and possibly alcohol intake enhance shortening of telomeres. However, strong evidence to support such an interpretation is hard to find. We therefore tested whether these lifestyle factors are associated with telomere length change in 4,576 healthy individuals from the general population.
Individuals had relative telomere length measured twice with a 10-year interval, and were then followed for mortality and morbidity for a further 10 years after the second measurement. We found change in telomere length to be more dynamic than previously believed, as we observed both shortening (in 56%) and lengthening (in 44%) among participants. Contrary to previous beliefs, we found telomere length change to be unaffected by lifestyle factors. Instead, we found the strongest association between past telomere length and age with change in telomere length over 10 years. Also, we found no association between change in telomere length and risk of all-cause mortality, cancer, chronic obstructive lung disease, diabetes mellitus, ischemic cerebrovascular disease, or ischemic heart disease.