These clocks promise to measure biological age and help identify anti-aging drugs, but there are lingering questions over their accuracy. Age is much more than the number of birthdays you’ve clocked.

How old are you, really? It seems like a simple question. It’s based on when you’re born. Yet we all know people who seem much younger than their chronological age. They have radiant skin and hair.

In 1997, Jeanne Calment passed away at the age of 122 and a half. The longest living human documented to date, she pushed the boundary of what was previously considered the maximum human lifespan.

This time a year ago, Steve Horvath was looking for pangolin DNA. The ancient scaly anteater would be a first for his collection, which was then about 200 mammals strong. “I didn’t have any of that ...