From Jayson Stark's column on the Athletic - https://theathletic.com/959684/2019/05/03/stark-useless-info-dept-vlad-all-over-edition/:
THE YOUNG AND THE TROTLESS – The Nationals had three players aged 21 or younger homer in the same game Sunday. As the Elias Sports Bureau was there to inform us, no team had ever had three teammates under age 22 homer in the same game before. So that was cool. But now let’s put that feat in a different light:
The Diamondbacks have had one player 21 or younger – Justin Upton – hit a home run in the history of their team. They’ve been around since 1998.
The Rockies have had two players 21 or younger – Troy Tulowitzki and Roberto Mejia – hit a home run in the history of their team. They’ve been around since 1993.
The Angels have had one player 21 or younger – some guy named Mike Trout – hit a home run in the last 34 years. We’re talking all the way back through 1985.
And then there are the Phillies. They’ve had two players 21 or younger, hit a home run at any point in the last 44 seasons – back through 1976. Those two? Scott Rolen and (of course) Luis Aguayo!
But the Nationals just had three guys that young homer in one game. Wow.