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.
Jay Dahl
Dahl wasn't the only rookie in the lineup for Houston that night; in fact, the entire Colts starting nine were first-year players (the only time this has ever been attempted by an MLB club -- at least after 1876, the year the first major league began):
Dahl got New York out in the first inning, but allowed three runs in the second and four more in the third before being removed by manager Harry Craft. (Danny Coombs and Joe Hoerner, who followed Dahl on the mound for Houston, were also making their MLB debuts.) The Mets went on to win, 10–3. It was to be Dahl's only major league appearance. (As of 2017, Dahl is the most recent MLB player under the age of eighteen.)