Telomeres are repetitive DNA sequences that cap the ends of chromosomes, protecting the genetic material from degradation each time a cell divides. They shorten progressively with each cell division, and were, for a period, treated as one of the most direct measurable signatures of cellular aging, which is part of why telomerase gene therapy has attracted attention as a longevity intervention (see the gene therapy entry).
The evidence picture has become more complicated since telomere length testing became commercially available. Measured telomere length varies substantially between cell types, between labs running the same test, and even between repeated tests on the same person, high enough measurement noise that a single test result is often not reliable at the individual level. Population-level associations between shorter telomeres and disease risk are real but weaker and less consistent than initially reported, and telomere length doesn't reliably outperform other biomarkers like epigenetic clocks in predicting mortality.
This entry is included deliberately, not to promote telomere testing as a reliable individual biomarker, but because it's one of the most commonly marketed longevity tests and readers deserve an honest evidence assessment: interesting biology, genuinely part of the aging story, but not yet a dependable standalone measurement the way epigenetic clocks or grip strength have become.