Most people in their 20s have all of their hair, few wrinkles, and lots of energy. But a new study from the University of California reveals they’re considerably less happy than older adults, despite those physical perks.

Researchers found that while cognitive and physical abilities fade in old age, mental health – including mood, sense of well-being, and ability to handle stress – improves until the very end of life.

The data was collected from a random sample of 1,546 people between the ages of 21 and 100 in the San Diego area. Each participant answered a series of questions about how happy and satisfied they feel, as well has how much depression, anxiety, and stress they experience.

"Participants reported that they felt better about themselves and their lives year upon year, decade after decade,” senior author Dr. Dilip Jeste, professor of psychiatry and neurosciences and director of the Center on Healthy Aging at UC San Diego, said in the report.

Jeste says the reasons for the improved sense of well-being in the second half of life aren’t clear, but notes that the results are “linear and substantial.”

Previous studies have reported a U-shaped curve of overall happiness across the lifespan, with declines from early adulthood to middle age followed by an improvement in later adulthood. That midlife dip did not materialize in Jeste’s snapshot of data.

Jeste observed the highest levels of perceived stress and symptoms of depression and anxiety among adults in their 20s and 30s participating in the study.

"This 'fountain of youth' period is associated with far worse levels of psychological well-being than any other period of adulthood," he said.

The oldest cohort had mental health scores significantly better than the youngest cohort, though the former's physical and cognitive function was measurably poorer than the latter's.

Jeste suggests the collective elements of wisdom in old age may be a factor. A number of studies have shown that older individuals tend to be better at emotional regulation and complex decision making. They also tend to experience and retain fewer negative emotions and memories.

Jeste says older people tend “not to sweat out the little things,” and realize that “a lot of previously big things become little.”