This article from the Employee Benefit Research Institute reviews the latest available data on the older population's income in the US and how it has changed over time. It also investigates how older people’s reliance these sources varies across demographic characteristics. 

In 2008, Social Security was the largest source of income for those currently age 65 and older, accounting for 39.8 percent of their income on average. Pension and annuities income was 19.7 percent, income from assets 13.0 percent, and income from earnings was 25.6 percent. Nearly all individuals (89.2%t ) age 65 and over were receiving income from Social Security in 2008, while 55.3 percent received income from assets, 35.4 percent received income from pensions and annuities, and 20.4 percent received income from earnings. Real median income of older people reached $18,001 in 2008, the highest point in the Census Bureau time series. The paper is available below:

Income of the Elderly Population Age 65 and Over