The last time participation in the labor force was that low was in 1978.
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The labor force participation rate in 2013 for Americans in their twenties hit the lowest level recorded since 1981, when the Bureau of Labor Statistics started releasing employment data on people in the full age bracket of 20 through 29.

The labor force participation rate for people ages 20 through 24—which BLS has been tracking since 1948—hit a 42-year low in 2013.