Have you ever met a younger person who seemed to be wise beyond their years? Or a 50-year-old who was stuck in the eternal temper tantrum of a terrible two? These are expressions of what some writers call your soul age. A young soul is typically concerned with success and building a career. A mature soul is more interested in relationships, and an old soul is more philosophically oriented.
Some of the literature links the theory to re-incarnation, which is not a belief I hold, but I did find the descriptions interesting.
You can Google soul age for more information if this is a topic that interests you. Here is an article about old souls, just in case you know any.