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.
Rafael Devers
In the postseason, Devers hit a two-run home run in Game 3 of the ALDS against the eventual World Series champions, the Houston Astros, giving the Red Sox their first lead in any game of the series. Devers became the youngest Red Sox player to homer in the postseason, and only the sixth player in MLB history to hit a home run in the playoffs before his 21st birthday, joining Mickey Mantle, Andruw Jones, Miguel Cabrera, Manny Machado, and Bryce Harper. In Game 4, Devers hit an inside-the-park home run, becoming the youngest MLB player (at 20 years, 350 days old) to hit one in the postseason. During the ALDS, Devers batted 4-for-11 (.364) with two home runs and five RBIs.