I don’t go that far back but in about 1951 I was given the opportunity to apply to be an altar boy. As I remember I was recommended by my cousin who was two years older and a fully proficient altar boy, given permission by my parents, interviewed by one of the priests, and accepted to begin training. There was a fair amount of Latin study/memorization involved, hours of practice and finally progressive levels of responsibility in the various liturgies.
There was an expectation that altar boys would be prime candidates for the priesthood, I don’t know what percentage actually went that route though. Neither my cousin nor I did although we both did seriously consider it.