For the 2014 campaign, the youngest player in the MLB is Dilson Herrera, according to STATS LLC. The 20-year old became the league's youngest when he made his late-season debut with the New York Mets.
The Colombian second baseman was called up by the Mets after an impressive season at Double A Binghamton, where he batted .340 with 10 homers, 48 RBIs, 17 doubles and 50 runs in 61 games, skipping Triple A ball altogether.
Dilson appeared in 18 games for the Mets, batting .220 with three home runs and 11 runs batted in.
The youngest player ever to appear in an MLB game was 15-year-old Joe Nuxhall, who made his debut on June 10, 1944.