Across the board, aging is the strongest risk factor for most non-communicable diseases such as cancer, diabetes, and Alzheimer’s. Most people will spend a significant fraction of their lives affected by conditions like these. By some estimates, nearly 95 percent of U.S. adults over 60 have at least one chronic condition, while nearly 80 percent have two or more. (Read how to take care of your brain as you age.)

Yet chronological age may not be a good indicator for defining old age, the new study says. Rather, individuals might enter old age at different times, according to their own perceptions.

And it’s true that no one wants to be old, especially as aging stereotypes have become more negative over the last 200 years—fueling what some call a worldwide crisis of ageism. Ageism, which can be as subtle as hiring a more youthful candidate or as blatant as disrespect, leads to social isolation, poor health, and even earlier death, according to the World Health Organization.

Becca Levy, a professor of epidemiology and psychology at Yale who was not involved in the new study, has spent much of her career untangling the downstream effects of age beliefs, which she detailed in her book, Breaking the Age Code.

“More negative age beliefs can lead to worse physical, mental, and cognitive health,” Levy says, whereas positive ones precipitate better health. One of Levy’s analyses estimates that a one-year cost of ageism in the United States totals $63 billion.