Midlife crisis starts when? Exactly when you are 44 years old, it looks like.

For a study by the University of Warwick and Dartmouth College in the US found the risk of depression was lowest in younger and older people, while the middle-aged years seemed to be associated with the highest risk for both men and women.

Data analysis on two million people from 80 countries found a remarkably consistent pattern around the world.

The only country which recorded a significant gender difference was the US, where unhappiness reached a peak around the age of 40 for women, and 50 for men.

Previous research has suggested that the risk of unhappiness and depression stays relatively constant throughout life.

However, the latest finding - of a peak risk in middle age - was consistent around the globe, and in all types of people, BBC reports.

Researcher Professor Andrew Oswald, an economist at the University of Warwick, said: "It happens to men and women, to single and married people, to rich and poor, and to those with and without children."

He said the reason why middle age was a universally vulnerable time was unclear.

However, he said: "One possibility is that individuals learn to adapt to their strengths and weaknesses, and in mid-life quell their infeasible aspirations.

"Another possibility is that a kind of comparison process is at work in which people have seen similar-aged peers die and value more their own remaining years. Perhaps people somehow learn to count their blessings."