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Between 1978 and 2002, it took approximately 5 years after leaving school for the first time before the average worker started a job that lasted 3 years.
The length of the transition varied by level of educational attainment.
Data from the National Longitudinal Survey of Youth 1979 (NLS79) show that the median high school dropout took nearly 11 years before starting a job that would last 3 years. In comparison, the median high school graduate took 6 years to start a job that would last 3 years. Those with a college degree settled into stable employment much more quickly; within a year and a half they started a job that would last 3 years.
In other words, the median high school dropout started a job that would last 3 years at age 29; the median high school graduate, at age 24; and the median college graduate, age 26.
These data are from the BLS National Longitudinal Surveys program. For additional information, see "The transition from school to work: education and work experiences," by Julie A. Yates, Monthly Labor Review, February 2005.
Bureau of Labor Statistics, U.S. Department of Labor, The Economics Daily, Transition from school to work quicker as education increases at https://www.bls.gov/opub/ted/2005/feb/wk4/art03.htm (visited March 27, 2023).