For centuries, sages have alluded to a richness in life's later years that is lost on the young. But only in the last decade have researchers begun to measure happiness across the life span and try to understand why older people tend to be so content.

Most scientists now think that experience and the mere passage of time gradually motivate people to approach life differently. The blazing-to-freezing range of emotions experienced by the young blends into something more lukewarm by later life, numerous studies show. Older people are more likely to focus on the positive, ignoring the negative.

In a study published in September in Psychological Science, neuropsychologist Stacey Wood and a collaborator recorded the brain activity of 63 adults, with a range of ages, who were shown a series of negative and positive images, such as dead animals or a bowl of ice cream. Older adults were about 30 percent less reactive to the negative images than younger adults.

"What we see is a real difference in how negative information is processed," Dr. Wood says.

Why people regulate emotions better as they age may be due to school-of-hard-knocks experience. The later stages of life offer more opportunities to avoid those parts that are stressful, Dr. Wood says.

A person's sense of time also may be a factor. Older people are aware that life will end – and, with a finite amount of time, they think it should be well spent. An appreciation of remaining time also leads older people to be more grateful, researchers say.

Nevertheless, individual temperament is the best predictor of happiness overall, Dr. Wood says. A happy child will probably be joyful decades later. The reverse applies, too.