Harper is 20, and Trout is all of 21. Barely old enough to drive, let alone buy a drink.
Machado's jersey ranked eighth, one spot behind Jeter, even though the Baltimore third baseman has spent less than a year in the majors. That didn't stop him from earning his first All-Star selection on his 21st birthday.
Machado was voted in by players, a significant sign of respect from his peers.
Well, mostly elders, actually. He certainly deserved it at a power-packed position after hitting 39 doubles in the first half, threatening the single-season record of 67 set by Earl Webb in 1931.
"In today's era, young dudes are getting better and more prepared to come up to the big leagues," said Orioles teammate Adam Jones, an All-Star himself. "It's just an improvement in the game. These young dudes are phenoms, and he's put his name up there.
"He's probably more mature than I am, and I'm 27."
Take a swing around the majors and you see it's not only Trout, Harper and Machado.
There is Miami rookie Jose Fernandez, a 20-year-old All-Star with a Cy Young future. Second-year shortstop Jean Segura in Milwaukee, who leads the NL in hits at age 23. Don't forget lefty Patrick Corbin (23), 11-1 with a 2.35 ERA for Arizona.
And of course, Harvey, the New York Mets ace who has 29 major league starts.
"For me, he's the best pitcher in the game," Diamondbacks outfielder Cody Ross said. "Not even just in the National League. He's really good. I faced a lot of those guys in the American League last year and I can't say that I saw anyone better than him.
"His mound presence is as good as you'll see."