I believe the youngest was Felix Heredia at 20 years old.........Miquel Cabrera was also 20 but i think Heredia was a "younger" 20. As far as the oldest, i'll have to take a pass on that one.....
Charlie Hough was the oldest players and Felix Heredia, by 11 days, was the youngest player to ever appear in a game for the Florida Marlins.
Heredia made his debut at 20 years and 52 days of age on Aug 9, 1996, in Florida, relieving fellow reliever Jay 'Molasses' Powell in the top of the eighth inning against the Mets. Heredia faced just one batter, Carlos Baerga, but recorded two outs as baserunner Bernard Gilkey was caught stealing on the pitch before Heredia coaxed a ground out from Baerga's bat.
Hough's final game in the majors was on July 26, 1994, about two weeks before the infamous strike. He was 46 years and 202 days old at the time. Hough started for the Marlins at home but was knocked from the box after retiring just one batter in the top of the first inning and allowing five runs to the Phillies. The last batter Ol' Charlie faced was Kim Batiste.
Thought I'd get some Miguel Cabrera answers, which would be close as he was 20 years and 63 days old when he made his big league debut in 2003.