We are centenarians

"We are the long-lived" is a brief title for a very important book, which suggests that its author is talking about his experience, as one of the group of long-lived people, that is, those who are destined to live a long life. The benefit of his referring to the "group" and not the individual is to emphasize that longevity is not an individual condition. Rather, it is the case of a group that includes large numbers of people, meaning that this dream is available to many, and those whom the book addresses can be members of that group.
The title, "We Are the Long-lived," suggests that the speaker is speaking from an experience that the book conveys to his reader. It also suggests that longevity does not mean the existence of that legendary drink called the elixir of youth, but rather it means following a healthy and sound lifestyle, whether in terms of nutrition, exercising, or following useful habits. Far from the opposite, we as Muslims believe that life is destined by God.
We deal with the topic of "longevity" as if it is not eternal immortality, but rather obtaining good health and a happy life, which means delaying the symptoms of aging, and this is exactly what the book aims to achieve.