Personality is traditionally understood as those aspects of yourself that don't change, but recent research has shown that this isn't strictly true. Although people don't generally undergo a complete personality change (apart from edge cases like major brain injury), people's personalities do shift over the lifetime. There are some predictable shifts, i.e. most people do become more introverted and more conscientious as they age. And then there are individual shifts that may have to do with changing careers and discovering a new side of yourself, or starting a family, or even adapting to a new culture.