Yes, on the job training, implying the person learning is being paid.Apprenticeships/OJT is the oldest form of training... not sure your serious with the who used them and when were they prevalent questions. In fact there was a time when it was all that was used...
You claimed there was a time when "young men" could "enter industries for little to no pay simply to "learn the business""... I asked what companies/industries this was common in, because I'm pretty sure skilled labor = paid for at least the last hundred years. I didn't realize you were referring to the Middle Ages, which are totally relevant to the labor situation in the US today