Some of the many other aging clocks developed since have become notable in their own right. Instead, it can be used to deliver a biological age that hints at how long a person might expect to live. Black-box clocksBut accuracy is a challenge for all aging clocks. No one needs an aging clock to tell them how many birthdays they’ve had. Therein lies the other central issue for scientists who develop and use aging clocks: What is the thing they are really measuring?