In the 1920s, the new production technologies and work simplification allowed automobile manufacturers to hire more and more women into light machine work and assembly jobs.
In addition, the new methods permitted more and more young workers to replace older veterans in the auto shops. The reason was simple—female workers cost much less than men, and young workers labored more vigorously at the new production lines.
The new technological system definitely favored the hiring of young workers in the auto factories. At Dodge, personnel managers “refuse[d] to hire men over 45.”
At the many General Motors plants in Flint, “Workers [were] scrapped at 40.”
With the onset of the Great Depression, too many men competed for too few jobs and automobile manufacturers took advantage of the glut in the labor market. The speed-up became unbearable for many workers, especially the older ones.
“Now,” the worker told the Automobile Labor Board, “the men that are operating these machines are most all young fellows.
They have to be, because I don’t know whether the old men could stand it, . . . and they all have reported that when they came in there to work that they had lost from 10 to 50 pounds.”
When a lay-off came during economic hard times, the automobile personnel offices only rehired the younger, more physically fit workers.
http://www.autolife.umd.umich.edu/Labor/L_Overview/L_Overview5.htm