The life expectancy of someone with untreated prostate cancer can vary greatly depending on the cancer's type, grade, and how quickly it grows. In many cases, especially with low-grade or slow-growing prostate cancer, men can live for many years without symptoms or major health issues. Some men with localized, early-stage prostate cancer may even die of other natural causes before the cancer becomes a serious threat. However, more aggressive or advanced prostate cancer can spread to other parts of the body, significantly shortening life expectancy if left untreated. It's essential to consult a urologist to assess the cancer’s behavior and determine the best course of action.