Mental illness is more common than most people expect. Everyone can suffer from mental illness at any time point of his life. A large survey in the UK found 1 in 6 people of working age suffer from a mental illness; mental illness is as common as asthma. Among those above 65 years old, one out of twenty suffers from dementia. Depression affects about 5% of men and 10% of women in developed countries. In 2000, the World Health Organization predicted that 20 years later, depression will become the second biggest burden of disease, second to ischemic heart disease. 

You can refer to the following website.

How common is mental illness?

The source of information is provided by the Hospital Authority Castle Peak Hospital