A work permit or employment certificate was required in most states resulted from child labor laws. You needed to prove your age, get your parent's approval and in some states, school approval. I believe a work permit is still required in many states.
In Oregon at the time a company could not hire you without a work permit which you had to apply to the state to get. I believe this was to prove you were indeed 14-years old or above.
I found a link; more info in the right hand side of the page.
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