Though a controversial subcategory of adulthood, humans are typically considered elderly, old, or senior citizens once they reach the age of 65 years old. Often understood as the final phase of adulthood in which a human nears the end of their life cycle, adults who fall into the category of elderly often experience many of the same symptoms characteristic of aging. Physical symptoms may include hair loss, hearing loss, and wrinkles in the skin. This is due to the fact that over time, the human body loses many of its regenerative abilities, making it more prone to injury, disease, syndromes, and sickness than that of younger humans.