Why your dog isn't the age you think

Usually aging is caused by the degradation of DNA over time, telomeres get shorter every time our cells divide, and eventually they get short enough that dna starts to... Unravel.... And genetic hiccups start occuring, and eventually things fail. Creatures that are biologically immortal dont experience that degradation, which is why we study them, to find out how they can prevent degradation in dna, as well as actually repairing genetic defects on their own, in the hopes of finding a way to transfer that ability to us for instance, making us biologically immortal, immune to aging, but other stuff could kill us, like being stabbed. But long story short, you quantify aging with the degradation of the telomeres in DNA. If you know how quickly those degrade in a particular living creature, you can use the state of degradation to determine how old it is. Hence why geneticists are the ones behind the study