At dawn on his 62nd birthday in 1366, Petrarch started a letter to his friend Boccaccio. “When you feel that you are old,” the Italian humanist told his fellow writer, “then and no sooner will you declare your old age.”

Why should we care about old age any more now than Petrarch did then? Demographics. Every day until 2030, about 10,000 Americans will turn 65. Between now and 2050, the number of Americans over 65 will double. By then, about 25 percent of the world’s population will be over 60.

Societies have long grappled with concerns about old age, though never on this scale. They sought to understand why people lived long lives, how they could be healthy in old age and what obligations society had to the elderly — precisely the questions we are pondering today.

Ancient cultures believed a long life indicated a good life, attributing impossibly long life spans to their favored leaders. Methuselah, who was granted the longest biblical life span at 969 years, outpaced other patriarchs like Adam, next in line at 930, or matriarchs like Sarah, who made it to a mere 127. Outside the Bible, we find a Sumerian King List that records monarchs like En-men-lu-Ana, who was said to have reigned for a spectacular 43,200 years. Two other Sumerian kings ruled for 36,000 years, two for 21,000 years and still two more at 18,600 years.

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