Cody Bellinger was born in 1995. Cody Bellinger made his MLB debut for the Dodgers on April 25—that's less than two months ago. Cody Bellinger has hit 21 home runs in his first 51 major league games. Cody Bellinger is good at baseball.
So, 21 homers in your first 51 games. Or 21 homers in your first 212 plate appearances. No matter how you put it, Bellinger has been insanely good. Coming into the season, he was highly regarded (#7 prospect according to Baseball America), but no one expected this. In 1500 minor league plate appearances, Bellinger had 65 home runs (4%). Compare that with 21 in 212 against major league pitching (10%).
So where does this start rank Bellinger all-time? 61 players have hit 20 home runs in their debut season. How many can you name??? Just kidding. Of those 61, the player with the least amount of plate appearances before Bellinger was Kevin Mass of the 1990 Yankees with 300. Who the heck is Kevin Mass? Obviously, Bellinger could hit five homers the rest of the season while raking up the plate appearances and drop on this list. But, right now he sits atop.
If Bellinger gets to 30 homers—by the All-Star break?—he'll be tied for 8th on the debut season home run list. A note: this is only for debut seasons, so it would leave off guys like Trout who came up for a brief stint the year before.
A few other notables Bellinger is likely to pass up soon: Giancarlo Stanton (22), Bryce Harper (22), Carlos Correa (22), Tony Conigliaro (24), Darryl Strawberry (26), Kris Bryant (26), Trevor Story (27), and Joe DiMaggio (29).