Aging and mortality are intrinsic to the human condition. Accepting our mortality as humans underpins our search for meaning in life and our quest for wisdom and maturity. But when the average age of The Rolling Stones exceeds that of the U.S. Supreme Court, it is clear that an adjustment to our concept of aging is warranted.

At least measured chronologically, everyone is aging and at the same rate — one year every year. But aging is so closely connected with our sense of impending mortality that it usually refers to end of life. This is evident in the widespread concern about an “aging society” defined in terms of a rising proportion of older people.

What Defines Being Old?

If aging is about being old what defines being old?

Two aspects tend to feature predominantly in our social discussions of age.

The first is biological and connected to declines in the physical and mental capabilities that circumscribe our daily lives.

The second is psychological and revealed in Cicero’s remark that “old age is the final scene…in life’s drama.” Stanford Center on Longevity’s Laura Carstensen’s Socioemotional Selectivity Theory posits, for example, that as we approach the end of life, our time horizons narrow. We focus on our most emotionally meaningful relations and activities.

These biological and psychological aspects of aging are deeply embedded. In practice, however, governments rely on more simplistic chronological measures such as the “Old Age Dependency Ratio.” This is a ratio of the number of old people relative to those of working age, where “old” begins at 65. That’s often mentioned in discussions of the solvency of Social Security.

When We Started Relying on Chronological Age

This reliance on chronological age is actually quite recent, though. Starting around 200 years ago, governments began to accurately record birth dates and then started to use chronological age to segregate society. Schooling became defined by age cohorts.

The apotheosis of this approach was the creation of a “retirement age,” broadly defined as 70 in 1908 and then as 65 in 1925. In response, individuals began to base their sense of age on their birth dates. Today, we think about age in terms of the number of candles needed for a birthday cake. (Did you know the Happy Birthday song didn’t start becoming popular until 1935?)

A focus on chronological age would be fine if its relationship with biological age were constant. But sustained improvements in life expectancy mean this isn’t the case.

What’s Wrong With Focusing on Chronological Age?

Over the course of the 20th century, life expectancy increased around 10 years for each generation. What this means in practice is that a 75-year-old today has the same mortality rate as a 65-year-old in 1950. So perhaps 75 really is the new 65.

Over the same period, the average age of the U.S. population has increased from around 32 to 38, while the average mortality rate has fallen by 13 percent. As a result, the average U.S. citizen has never been older, but also has never had so long left to live.

This decline in mortality rates has implications for the understanding of old age. If we are, on average, healthier at each chronological age, then biological aging is occurring more slowly. We are, in effect, younger for longer.

This increase in life expectancy has already changed the way we experience the earlier phases of the lifecycle. Adolescence now extends well into one’s 20s. Young adulthood these days is characterized by a period of exploration and discovery, free from traditional grown-up responsibilities such as marriage and parenthood.

New Thinking on Lifestages

Following on this logic, should our 40s and 50s also become a time of reinvention and rediscovery in anticipation of a longer second half of life?

If we are healthy in our 70s and 80s, should this be a time of more work, deepening engagement in civil society and/or greater leisure and play?

In this era of a “new old age,” we will have a longer time to recreate our own identities, avail ourselves of a wider set of opportunities and come to terms with past mistakes and misfortunes.

There is a further reason why we need to move away from the dominant chronological concept of age.

The real truth about aging is diversity. You may be like the Frenchman Robert Marchand, still breaking cycling speed records at 105. Or you may end up in a wheelchair by 50. This diversity of experience in what it means to be old will become all the more apparent as more people live into old age.

How Shifting to Biological Age Helps

A shift from chronological to a biological sense of age undermines lazy, age-based stereotypes and helps us to understand better how our own efforts may influence the aging process. This shift also forces governments and corporations to rethink education, retirement and pension policies, as well as employment practices.

Our relationship with time changes when we have more of it. Lacking role models for how to live such long lives, we are —  all of us — currently engaged in a huge, new and long-lasting social experiment. This moment provides an opportunity to free ourselves from the numerical determinism of chronological age and revisit a more humane concept based on an individual’s physical and mental characteristics.

To return to Cicero, only then we will approach that final stage with a sharper sense of who we are.