Telomere
Protective DNA caps on the ends of chromosomes that shorten each time a cell divides.
Telomeres are repetitive DNA sequences capping the ends of chromosomes, protecting the coding DNA from degradation and from being mistaken for a broken strand. They shorten a little with each cell division, because the replication machinery cannot copy the very end of a strand.
When they become critically short the cell stops dividing and enters senescence. That limit is one of the reasons cells cannot divide indefinitely, and it is a genuine part of ageing biology.
Telomere length is associated with age and, in large studies, with mortality and some diseases, which is why it became a popular biomarker of ageing and the basis of a consumer testing industry.
That industry runs well ahead of the evidence. Measurement varies substantially between laboratories and methods, individual variation at any given age is enormous, and the change over a year is small relative to that measurement noise. A single consumer result tells you very little about your own ageing.
The direction of causation is also unsettled. Shorter telomeres may reflect accumulated stress and disease rather than driving it, and the enzyme that lengthens them, telomerase, is active in most cancers, which makes lengthening them a considerably less obviously good idea than the marketing implies.
Supplements sold as telomerase activators rest on cell and animal data, and no human outcome trial supports them.
In practice, treat telomere length as interesting population biology and a poor personal test. If you want a biological age estimate that shows its working, our phenotypic age calculator uses standard blood markers with published coefficients instead.