Researchers from the Substance Abuse and Mental Health Services Administration in the U.S. have been analyzing data from the 2007 National Survey on Drug Use and Health involving around 45,000 people. They found that 7.5% of people suffered from at least one bout of major depression in the last year. 18-25 year-olds were most likely to suffer from depression (8.9%), more than those in the 26-49 age group (8.5%) and those over 50 (5.8%). 14.2% of Americans who described their health as 'fair' or 'poor' were depressed, compared to only 4.3% of healthier people. Only 64.5% of people who suffered from depression actually got treatment. Of those who did not get treatment 43% said cost was the main reason, 29% said they could deal with their depression on their own and 18% said that they did not know where to turn for help.
You can find out more about the levels of depression in the U.S. at