Depression is most likely to affect people in middle age, new research suggests.
Experts from the University of Warwick and Dartmouth College in New Hampshire looked at data from two million people in more than 70 countries.
They found happiness and depression follow a U-shape over a person's lifetime, with the peak age for depression in the UK being reached at around 44.
But pensioners can experience the same levels of happiness at 70 years old as youngsters aged 20, the study published in the journal Social Science & Medicine claims.
Professor Andrew Oswald, from the University of Warwick, said: "Some people suffer more than others but in our data the average effect is large.
"It happens to men and women, to single and married people, to rich and poor, and to those with and without children. Nobody knows why we see this consistency.
"What causes this apparently U-shaped curve, and its similar shape in different parts of the developed and even often developing world, is unknown.
"However, 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 cheerful people live systematically longer."
"This is the time to talk more and educate people about depression. This system concerning cuts affecting people of incapable allowance which supposed to be gradually introduced but just introduced sharply, this will bring more people to hospital. Government has to employ more healthcare professionals" - John Smith, London
"Your metabolism slows and you cannot enjoy the good things in life without putting on weight. You are likely to have a teenager in the house, not easy to deal with" - Name and address supplied
"Your features change, body changes, you can't do as much as you could when you were in you teens. Things you have done take their toll." - Name and address supplied