Professor Oswald said for the average person, the dip in mental health and happiness comes on slowly, not suddenly in a single year.
Only in their 50s do most people emerge from the low period.
"But encouragingly, by the time you are 70, if you are still physically fit then on average you are as happy and mentally healthy as a 20-year-old.
"Perhaps realizing that such feelings are completely normal in midlife might even help individuals survive this phase better."
Marjorie Wallace, chief executive of the mental health charity Sane, said: "This study raises intriguing questions about the processes that lead to depression in mid-life, as well as indicating what a common experience it is worldwide.
"Depression is a complex and challenging condition that remains poorly understood, with as many as one in ten people with severe depression taking their own life.
"We welcome any scientific contribution to our understanding of this illness, particularly if the research can aid the development of better treatments, both therapeutic and pharmaceutical."
Andy Bell, of the Sainsbury Centre for Mental Health, said mental health problems were extremely common - but he stressed they could occur at any time in life.
Source-ANI
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