The youngest player to ever make an appearance in a major league game was Cincinnati Reds pitcher Joe Nuxhall. On June 10, 1944, he pitched two-thirds of an inning in his debut, giving up five runs on two hits and five walks. Nuxhall was just 15 years, 10 months, and 11 days old.
A year earlier, Carl Scheib set a record that still stands in the American League, pitching in the second game of a doubleheader for the Philadelphia Athletics on September 6, 1943. Scheib was 16 years, 8 months, and 5 days old.