Mike Trout
On April 17, 2015, Trout became the youngest player in MLB history to reach 100 home runs and 100 stolen bases. He was 23 years and 253 days old when he reached the milestone, passing the previous record-holder, Alex Rodriguez, who had achieved it at the age of 23 years and 309 days in 1999. Trout led off the 2015 MLB All-Star Game with a home run, becoming the fourth player in All-Star Game history to do so. For the second year in a row, Trout won the All-Star Game MVP Award, becoming the first player ever to win it in consecutive years. On September 22, Trout hit his 40th home run, becoming only the second Angels player to hit 40 home runs in a season. Trout led the AL in WAR for the fourth straight year.
Sean McDonough
Technically, Vin Scully, who was 25 when he called his first World Series in 1953, is the youngest man to ever do play-by-play for a World Series. However, unlike Sean McDonough and later, Joe Buck, Scully was there as a representative of the Brooklyn Dodgers. The policy of World Series broadcasters at the time allowed representatives of the participating teams to do alternating play-by-play on the national television broadcasts instead of an actual network employee (as was the case for Scully when he was NBC's lead baseball play-by-play man from 1983 to 1989).