The term "prime years" refers to a period in one's life when they are at their peak physically, mentally and professionally. This is usually between the ages of 25-40 or so, depending on individual circumstances such as genetics, lifestyle choices, and health habits. During these years, people tend to have more energy, strength, and endurance than at any other time in their lives. They are also typically at their most productive professionally, with the greatest potential for achievement and advancement. In summary, "one's prime years" refers to a period of life when one is physically, mentally, and professionally at their best.