They weren't. The highest opiate addiction rates may have been after the Civil War, where many soldiers had morphine habits, but that still represented a small proportion of the population. Incidentally, there wasn't much of a social stigma against it at the time, as it was viewed as "the soldier's disease" rather than a moral failing or weakness. I doubt that the average victim of an industrial accident in the early 1900s had the wherewithal, or desire, to consult a doctor or pharmacist, let alone afford an admittedly cheaper habit compared to today's standards.
Even when cocaine was legal, the majority of the population was not addicted. It's probably a more recent phenomenon that Americans have the attitude that medications can cure their ailments, risk free; certainly, the advent of televised pharmaceutical ads have increased the receptiveness of the average person toward taking medication, even for pain.
TL;DR, I don't think there was ever a "generation of junkies."
Also, I don't think the DEA has standards for addiction. Addiction is viewed as a medical/psychological problem, and as such, the only criteria for addiction that I can think of that has been formalized and accepted by professionals are the DSM-IV definitions of substance abuse and substance dependence.
