To start with, we need to understand why we age in the first place. Aside from the existential dread it may cause, ageing is a complex process whereby there is a progressive loss of the body's ability to maintain itself. Your body runs on finely tuned chemical machinery, proteins, and the cell's instruction manual - DNA. Like all machinery, proteins break, malfunction and accumulate errors over time. Highly reactive chemicals called free radicals are produced by metabolism, and through exposure to the environment, such as UV rays, they can damage proteins and DNA. To prevent this, cells constantly maintain order and cleanliness. The cell's maintenance mechanisms repair damaged DNA, discard old proteins, and signal the cell to make new proteins. Eventually, the cell becomes too damaged and cannot work anymore. The body makes more cells to replace these old ones. However, cells have an internal limit - defined by their DNA - on how many times they can divide, meaning that we can't keep creating new cells forever. Over time, with fewer new cells to replace old ones and with the damage piling up, it becomes difficult to maintain our youth. This is part of what leads to the physical manifestations of old age, as well as some common diseases, such as cancer and diabetes. So, logically, if we could have more young cells and maintain our existing cells better, we wouldn't get as sick and broken down when we're older.