Nearly a quarter of people aged between 65 and 74 were in America’s labor force in 2006 - up from a fifth in 2000. By 2020, it’s thought that one in three of America’s workers will be older than 50 and the rate of increase of the gray population in the labor force will be four times greater than the rate of overall labor force growth.

Currently the highest rates of older workers are found in South Dakota, Nebraska and Washington D.C., all with about one-third of 65 to 74 year olds in the labor force.

Older workers will be vital in the American economy because the post-baby-boom generation, born after1964, is about 16 percent smaller in size than the baby boom generation.

Among the 20 largest metro areas, Washington, D.C., had the highest percentage of people in the labor force in the 65 to 74 age group (31.8 percent). Others with high percentages include Boston (28.1 percent), Dallas-Fort Worth (27.9 percent), Minneapolis-St. Paul (27.4 percent) and Houston (26.5 percent).

According to census numbers, states that currently have some of the lowest rates of older workers in the labour force include West Virginia, Michigan and Arizona.