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.

Those mythical ages may have been a sign of goodness or strength, but not necessarily happiness. Petrarch used the Bible’s oldest person to conclude that long life did not necessarily bring joy and satisfaction. “If living long made us happy, the happiest of all would have been Methuselah, which no one has ever said or believed,” he wrote. “Not the quantity, but the quality of a life must be considered, nor do the length of years matter but the splendor of the deeds, and especially the end of life.”

This conflation of life span and virtue reached across continents. A Taoist ruler called the Yellow Emperor was a fabled Chinese figure from around 2700 to 2600 B.C. He wondered in a dialogue with a heavenly sage why no one in his day lived as long as their ancestors had: “I hear the ancient people did not decline in health even when they were over one hundred years old. However, the present people become decrepit at just over fifty. Why is this?” The answer: “Because the present people do not pay attention to natural law [and instead] pursue pleasure and exhaust themselves before they accomplish their natural life span.”

On the Christian side, a medieval Benedictine abbot named Engelbert of Admont argued that human beings had lived long through a combination of God’s will, healthy plants, and pure air and water. But then life spans shortened because people gave in to lusts and desire.

As the Scientific Revolution proceeded, some theorists looked for more technical and less moral interpretations of declining life spans (at least compared with the legends of hundreds and thousands of years). Robert Hooke, a 17th-century English natural philosopher, proposed that friction made the speed of Earth’s rotation slow down over the centuries, leading to modern years being longer than those in prehistoric and biblical eras. Thomas Burnet, a 17th-century chaplain to Britain’s King William III, offered a geological explanation, but one still grounded in a belief that the story of Noah’s ark was historically true: The flood shifted Earth’s axis. Before this happened, it was spring all the time, so it was easier to live longer; after the flood’s axis shift, three more seasons came into being and made life harder.

Embedded in the conversation about life spans is a concern about the duties a society has to its elders, down to whether such duties are mandated. Yahweh lays down the law in the Ten Commandments, “Honor your father and mother.” Notably this commandment continues with an incentive not often mentioned: We should honor our parents “so that your days may be long in the land that the Lord your God is giving you” (Exodus 20:12). The young should honor the elderly, if only because the young’s own reward will be long life.

The Koran likewise commands, “The Lord has decreed … that you be kind to parents. Whether one or both of them attain old age in your life, say not to them a word of contempt, nor repel them, but address them in terms of honor.” Lest a Muslim child fail to comply, the Koran reminds her of her debt to the parent: “And, out of kindness, lower to them the wing of humility, and say: `My Lord! Bestow on them your mercy even as they cherished me in childhood.’” (Sura 17:23-24).

Ancient Greeks apparently had to be goaded into caring for the elderly by a law forcing adults to be their parents’ caretakers. Any man running for civic office was checked for compliance; a citizen could lose his rights if he didn’t fulfill this obligation.

More positively, in the Chinese “Xiaojing,” an ancient manual on how people should honor their elders, filial piety is foundational: “In serving his parents, a filial son renders the utmost reverence to them while at home. In supporting them, he maximizes their pleasure. When they are sick, he takes every care. At their death, he expresses all his grief. Then he sacrifices to them with full solemnity. If he has fulfilled these five requirements, then he is truly able to serve his parents.”

Caring and respecting is good for the older people, clearly, but it’s also recognized as an honorable deed for those doing the caring and respecting in any historical period.

It is now our society’s turn to plan how we care for our quickly aging populations. There are practical consequences. We need to recognize the staggering economic and social implications of our rapidly expanding senior population for national aid programs such as Medicare and Social Security, as well as local programs such as Meals on Wheels, housing for the elderly and food pantries.

But advice from ancient and medieval sources goes deeper: Our senior citizens are people with gifts still to give. As they reap the rewards of their well-lived lives, they can also still sow their own lessons if we connect generations. Pope Francis likes to give young people a specific homework assignment. “Speak to your grandparents. Ask them questions,” the pope told them in 2016 two days after his 80th birthday. “They have the memory of history, the experience of living, and this is a great gift for you that will help you in your life journey.”

In our religious and community centers, our schools and our nursing homes, programs can encourage interaction among the young and the old and establish mentoring programs for the recently retired to connect them with apprentices in unions, local community colleges and universities, and other professional training settings. Grandparents can tell their family histories to middle- and high-schoolers with recording equipment for oral history archives. Documenting their experiences illuminates how individuals grappled with the last century’s massive social and technological changes. By asking questions — What was it like to grow up poor in the Depression? To learn about the Holocaust? To break gender, class and racial barriers? — we can learn how to approach similarly challenging and controversial issues today.

By being more generous and intentional about our obligations to our senior citizens, we can also bestow a gift to our children that the Greek historian Diodorus Siculus understood in the 1st century B.C. when he said, “Knowledge of history … endows the young with the wisdom of the aged.”