In 2018, a 69-year-old Dutch businessman started a legal battle to try to change his age: he wanted to shave off 20 years by claiming that, according to his doctors, he had the body of a 49-year-old man. Our chronological age—the years we have lived—can differ from our biological age, which measures the degree of aging of cells, tissues and organs. As life expectancy increases, multiple researchers are exploring the potential of epigenetic clocks to understand aging and prevent disease.