Longevity is how long a person or creature has to live as far as an average lifespan. The Bible tells us that humans were originally intended to live forever, but when sin entered into the world, death was introduced, and man was given a lifespan of several centuries for the first several generations. When the earth became corrupt because of sin, the average lifespan was gradually reduced to 120 years, then 70 to 80 years, then 50 years at the time of Jesus Christ's first coming. As the scientific revolution brought about better hygiene and medication, the human lifespan expanded itself again to 70 or 80 years.

In the Left Behind series, prior to the Millennial Kingdom, a human being's lifespan was on the average 80 years, with women living longer than men. With the introduction of the Millennial Kingdom, naturals who have accepted Jesus Christ as their Lord and Savior were allowed to live to the end of the Millennium, thus their lifespans were extended to that point. However, those that refused to accept Jesus as their Lord and Savior were only allowed to live 100 years and then would "die of unbelief", usually suddenly, and be sent to Hell, where they would remain until the Great White Throne judgment at the end of the Millennium, where they would be sentenced to the Lake of Fire.

In his speech to the Children of the Tribulation, Noah said that he was given a glorified body, which set him back midlife to when he was 500 years old. He said he was spry and had three sons at that time. Thus, the rate of aging in the Millennial Kingdom would be such that one who has lived for five hundred years would have similarly physiological vitality as one who is in their mid to late thirties. In the Millennium Kingdom, those who were below the age of 100 were essentially regarded as adolescents. Kenny Bruce Williams and Ekaterina Risto were in their nineties when they met each other, but they were not wizened geriatric cases and socialized more like young adults. Noah said that the cause of the reduced senescence in the Millennial Kingdom was the return of a vapor canopy, which existed during the antediluvian time of the patriarchs, that blocked "the most harmful effects of the sun"

A calculator to figure out actual age from apparent age for people born in the Millennial Kingdom is available at https://emlia.org/pmwiki/pub/web/LeftBeyond.Aging.html