If you want to learn a fish's age, simply take one of its scales and count the number of bands radiating from its center.
For trees, you can count their rings.
For narwhals, their tusk layers.
And for blue whales, their layers of earwax.
When it comes to humans, scientists have yet to find any visible traits that mark our age with anywhere near the same specificity.
But in the past few decades, they've discovered small, invisible markers hidden within the body that do change over time, and they may hold even more information about our health, history, and future.
The first of these markers was discovered in the 1990s.
Telomeres are repetitive sequences of the ends of DNA strands that protect chromosomes from fusing together.
However, each time a cell divides, part of the telomere is cut off.
These small losses can add up over time, and once telomeres get too short, cells lose their ability to replicate and eventually die.
