As cells age, they become larger and less capable of reproduction, undergoing changes that eventually result in the loss of cellular identity. For years, scientists have argued about why these changes occur, but popular opinion has coalesced around nine “hallmarks of aging.” But are these hallmarks a cause or a symptom? In his new book, Lifespan, David Sinclair has proposed a radical new theory of aging, suggesting that the aging hallmarks are symptoms of epigenetic changes that occur over time, but that the original genetic code remains very much intact. If we can reset the code, can we slow, stop or even reverse aging? Sinclair believes we can.