Probably not a whole lot different than if you make them have 100 games at three or four minor league levels. You're not going to take most teenagers and make them 600 PA regulars. You break them in slowly, platoon them, play them in spots where they have more of an impact.
In the 1930s there were 121 seasons of players up through age 20 in the majors, with just 16 teams.
1970s: 156 (free agency hit here, but took awhile for the implications to be realized)
It's not just service time, there are other factors like quality of play. But prior to free agency and the importance of service time players were routinely given minor league apprenticeships that would be considered borderline criminal today.
Brooks Robinson was in the majors at 18. And not because he shot through five levels of minors, he had 400 PAs in Class B ball, which is kind of like Frederick today.
Palmer was mostly pitching relief in the majors at 19, after 19 starts with Aberdeen (ND) in Class A Ball. Frank was in the majors at 20 after 1.5 seasons in class A, and eight games in AA. Dave McNally's age 20 season was 7-8, 4.58 in the majors, after debuting at 19. Boog was in the majors at 19. Milt Pappas made four starts at 18. Jack Fisher pitched 27 MLB games at the age of 20.
Most of those guys had good careers, many of them great.