During the time of WWII there were some exceptions: For example on the PRR. That railroad did not have stokers on the 4-6-2's and they had many long fast runs. They hired boys as young as 16-17 and would use two of them as fireman on those trains simply because one person could not do the job. Of course the manpower shortage of WWII caused that extreme change. When hired, they were immediately put on trains with experienced fireman to learn the trade. From what I was told the turnover rate on the young men was very high.