Toolshed: Will Acuña be MLB's youngest?

By getting called up to the Majors at 20 years, 128 days old on Wednesday, Braves outfielder Ronald Acuña Jr. became the youngest player at the game's highest level this season. It's not even close.
The previous holder of that title was Acuña's Atlanta teammate Ozzie Albies -- the only other 2018 Major Leaguer who was born in the calendar year 1997. (Victor Robles is the only 1997-born player to ever appear in the Majors, but he's only appeared for Triple-A Syracuse this season.) But Albies was born in January, and Acuña was born in December. The second-youngest player in the Majors before Acuña showed up was Gleyber Torres, who was born a full year and five days before MLB.com's No. 2 overall prospect.

But what are the chances the 20-year-old slugger will be the youngest player in the Major Leagues for the duration of the 2018 season? As it turns out, they're pretty good, although there are a couple reasons he may not be.
First, let's look at the youngest players to play in the Majors over the 10 seasons before this one, along with their birthdays and ages in those seasons. (Ages listed correspond to those of the players at midnight on June 30th of that year.)

Any group that includes Trout, Kershaw and Bumgarner is a pretty phenomenal one. It also makes sense that this would be an impressive list. Talented Major Leaguers typically show their tools early, leading to aggressive pushes by their respective organizations. Sometimes -- as was the case with that trio -- it works out. Sometimes -- as with Profar and Urias -- injuries get in the way. 
While no one can say for certain what Acuña's future holds, he at least fits in the above table quite nicely. He isn't the youngest player to play in the Majors over the last 11 seasons -- that's Profar, who turned 19 just before his debut season in 2012 -- but he is only four months removed from his 20th birthday, placing him square in the typical age range for this group. 
But beyond a recent historical analysis, there are a few factors exclusive to 2018 that could point to the toolsy outfielder remaining the youngest throughout the rest of the season.

The case for Acuña

Starting out with Triple-A Gwinnett, Acuña was the youngest player on an Opening Day roster in the International League, and in the three weeks since that day, no younger player has stepped to the plate or taken the mound. The closest in age to Acuña in that specific circuit were, unsurprisingly, his Gwinnett teammates Mike Soroka and Kolby Allard, who are both about four months older than him. No one else currently in the IL is in the midst of their age-20 season. 
The Pacific Coast League has its own can of worms. There have been three players younger than Acuña to play in Triple-A's western circuit, but none is immediately big league-bound. Cesar Izturis Jr. (Tacoma), Sebastian Ochoa (Tacoma) and Cristian Inoa (Round Rock) have combined for just 10 plate appearances among them in the PCL and for good reason. All three were brought up from extended spring camp to fill an emergency role. In fact, Izturis and Ochoa had never played above the Dominican Summer League before briefly joining Seattle's Triple-A affiliate. Inoa spent last season at Class A Short Season Spokane. 
If there's going to be a younger player to play in the Majors than Acuña, that player isn't one step away right now.
But what about one roster move away? No dice there either, it appears. Acuña also happens to be the youngest player currently on a 40-man Major League roster. That makes sense in that really young players are only added when they're Major League-ready; they're not stashed on the 40-man for Rule 5 Draft protection until they're usually too old to be the Majors' youngest player.
So, there aren't any immediate candidates to unseat Acuña from the youngest Major Leaguer throne. But that doesn't mean the game is completely out of candidates.

The potential cases against Acuña

In fact, there are five current Double-A players that are younger than Acuña. On the one hand, the fact that there are only five Double-A players younger than an active Major Leaguer is astonishing. On the other hand, all five are incredibly talented in their own right and are well known to prospect fiends. Let's start with the youngest (an obvious candidate) and move to the oldest:
Vladimir Guerrero Jr., 3/16/99 birthday: This should have been everyone's first guess. The 19-year-old third baseman became the top prospect remaining in the Minors with Acuña's ascension. While it may seem crazy to suggest a player who won't turn 20 until next March can crack the Majors soon, everything should be on the table with the way Guerrero has taken to Double-A. Toronto's top prospect entered Thursday with a .353/.407/.529 line, two homers, six doubles and a 7/7 K/BB ratio through 17 games with New Hampshire. The native of the Dominican Republic has hit everywhere he's played in the pros -- he has a career slash line of .309/.403/.478 -- and he's only getting better as he's maturing physically and growing more accustomed to advanced pitching. Josh Donaldson's spot at the hot corner north of the border might be the biggest roadblock to keeping Guerrero out of the Majors, because it's becoming clear he'll pass any hitting challenge.
Fernando Tatis Jr., 1/2/99 birthday: San Diego skipped MLB.com's No. 8 overall prospect over Class A Advanced last season, had him finish up with Double-A San Antonio and returned him to the Missions to begin 2018. Unlike Guerrero, Tatis hasn't been hot out of the gate, batting .188/.195/.363. But he's said that he's not worried -- he also hit just .230 in April last year -- and backed it up with a two-homer game on Tuesday. Once his combination of power and speed starts to flourish in the Texas League, the right-handed slugger could be aggressively promoted again. The Padres have already been willing to push Joey Lucchesi and Eric Lauer quickly to The Show this spring, and neither have the prospect status of Tatis, though they are a few years older. A cup of coffee in September shouldn't be ruled out.
Side note: If either Tatis or Guerrero make the Majors in 2018, it would mean the 1998 year would be completely skipped over in the "Youngest Major Leaguer in a given year" discussion. The last birthyear that happened to was 1992, but don't cry for it; Bryce Harper, Mookie Betts, Manny Machado, Aaron Judge and Kris Bryant were all born then.
Keibert Ruiz, 7/20/98 birthday: Because they have to balance their offensive work and relationships with pitchers, catchers aren't usually pushed incredibly quickly. But the Dodgers have put a lot of trust in their 19-year-old, switch-hitting backstop by starting him out at Double-A Tulsa after only 38 games at Class A Advanced Rancho Cucamonga last season. Ruiz has handled the assignment well, hitting .286/.333/.446 with three homers in 13 games while throwing out four of eight attempted basestealers. He is sharing time behind the plate with Will Smith, however, and it's the 23-year-old who's more likely to get the bump between the two. Ruiz making the Majors in 2018 is a big stretch, but anyone who begins at Double-A is at least a candidate.
Bo Bichette, 3/5/98 birthday: The Blue Jays have tied Guerrero and Bichette together since Opening Day 2017, but in the spring, director of player development Gil Kim said there very well could come the day when one jumps in front of the other. Given the way they've each started Double-A, Guerrero would have the inside track, but Bichette has done plenty to hold his own. MLB.com's No. 13 overall prospect is hitting .303/.376/.434 with three triples, four doubles and seven steals in 17 games with New Hampshire. There's still the possibility the shortstop with a 70-grade hit tool could continue to grow offensively while Guerrero falters for the first time. A middle infielder, Bichette doesn't have someone the caliber of Donaldson blocking him from reaching Toronto, either. It takes some squinting to see the 20-year-old make the Majors in 2018 without Guerrero doing the same, but not too much.
Jose Suarez, 1/3/98 birthday: The Angels' No. 12 prospect is by far both the least prominent prospect on this list and the least likely to crack the Majors, but he is at Double-A and younger than Acuña. The 20-year-old left-hander is pitching for an affiliate in April for the first time ever, having opened in short-season ball each of the previous three seasons. But he is already moving quickly, jumping from Class A Advanced Inland Empire to Double-A Mobile. He's got an impressive changeup for his age and special control, having struck out 29 and walked only two in 17 2/3 innings between the two levels so far, and it's possible the Halos make him this year's slightly older version of Urias and utilize his pitchability in their hunt for a postseason spot. But Suarez has also yet to throw more than 72 2/3 innings in a regular season. He fits the bill, but consider his candidacy an extremely dark-horse one for now.
To illustrate just how limited the younger-than-Acuña pool is, here are the rest of MLB.com's top-100 prospects who are younger than the Braves outfielder but are extremely unlikely to debut in the Majors this summer:
MacKenzie Gore, Royce Lewis, Sixto Sanchez, Juan Soto, Hunter Greene, Leody Taveras, Adrian Morejon, Ian Anderson, Matt Manning, Jo Adell, Heliot Ramos, Shane Baz, Jay Groome, Kevin Maitan, Mickey Moniak, Anderson Espinoza