Aging is surprisingly ill-defined. Scientists don’t agree exactly why we age or how aging evolved. Current ideas include an increased chance of dying, loss of function, accumulation of damage over time, continued development, age-related changes, or now, an increase in biological age as measured by epigenetic clocks. Despite the lack of consensus, “late in life, they all apply, and they all go hand in hand,” says Harvard biomedical researcher Vadim Gladyshev. Still, “there must be one, most important feature that defines aging, but there is currently no consensus on what this feature is,” he adds.