The Washington Nationals outfielder has already been elected to two All-Star Games and won the 2012 NL Rookie of the Year award. He has become one of the top players in the game, and the world has taken notice.
The 20-year-old phenom could potentially still play for another two decades, but he isn't wasting time, as he's already tearing things up. In fact, he ranks third all-time for most home runs by age 21.
Harper's incredible performance got us at Bleacher Report thinking about the best players in MLB history who were 21 years old or younger.
Check it out.
RF Mel Ott, Indians
CF Andruw Jones, Braves
LHP Frank Tanana, Giants
Team: Washington Nationals
DOB: October 16, 1992
Best Season: 139 games, .270 BA, 22 HR, 59 RBI (2012)
Notable Honors Before Age 22: Two-time All-Star; 2012 NL Rookie of the Year
After being taken first overall by the Nationals in the 2010 MLB Draft at the age of 17, Harper was called up to the majors in 2012 at the age of 19.
Harper quickly impressed with the bat and with his athleticism in the field. He beat out Wade Miley for the Rookie of the Year award despite being six years younger than Miley, and he hasn't looked back since.
Harper is on pace to have an even better 2013 season, as his numbers spread over a 139-game period (the amount of games he played in 2012) would lead him to have 32 home runs and 70 RBI, not to mention that he's elevated his slugging percentage by 49 points.
Harper is still getting better, and by the time he turns 22, he'll have earned a place much higher on this list. However, for now he's forced to settle for No. 10.
Team: Detroit Tigers
Position: RF, 3B
DOB: December 19, 1934
Best Season: 152 games, .340 BA, 27 HR, 102 RBI (1955)
Notable Honors Before Age 22: Two-time All-Star
Al Kaline made his MLB debut on June 25, 1953 at the age of 18. He quickly learned the ropes, and during the 1955 season, he broke out and led the MLB in batting average.
In 1955, Kaline started the season at 20 years old and turned 21 halfway through. He was one of the youngest players in the league, but he was already one of the most dominant, combining his ability to hit for contact with some serious power.
Kaline would finish second in MVP voting that year and again in 1963. However, he never won the coveted award.
After a terrific start, Kaline went on to have a long, successful career that was highlighted by 15 All-Star Game appearances and 10 Gold Gloves. He was also elected into the Hall of Fame.
Team: Houston Astros
Position: CF, 1B
DOB: February 25, 1951
Best Season: 139 games, .320 BA, 22 HR, 82 RBI (1972)
Notable Honors Before Age 22: All-Star; Gold Glove
Cesar Cedeno was one of the great five-tool players in baseball's long history. Not only did he put up fantastic hitting numbers at the age of 21, but he also stole 55 bases and led the MLB in doubles (not to mention his Gold Glove award).
Unfortunately, Cedeno peaked early, as his .320 batting average was a career-high, and he only played until he was 35. He was only elected to four All-Star Games, as his decline came much earlier than anyone had hoped for.
However, there's no denying that Cedeno was one of the best young players in history, and if he had continued his torrid pace throughout his career, he could have ended up in Cooperstown.
Team: Milwaukee Brewers
Position: 1B, 3B
DOB: October 13, 1931
Best Season: 157 games, .302 BA, 47 HR, 135 RBI (1953)
Notable Honors Before Age 22: All-Star
Eddie Mathews entered the league at the age of 20, and in his second season, he casually slugged .627 and hit 47 home runs, which was the most by a player age 21 or younger at the time.
The reason Mathews shows up on this list is not because of his contact (he hit over .300 at age 21), but because of his overwhelming, undeniable power.
Mathews could take a ball deep like no other youngster his age, and no one has been able to recreate what he did in 1953. Heck, only one other player has even hit 40 homers at 21 or younger.
After finishing with 512 career dingers, Mathews is now enshrined in the Hall of Fame.
Team: Seattle Mariners
DOB: November 21, 1969
Best Season: 154 games, .327 BA, 22 HR, 100 RBI (1991)
Notable Honors Before Age 22: Two-time All-Star; two-time Gold Glove; Silver Slugger
Ken Griffey Jr. could've been the all-time leading home run hitter in MLB history if he never got hurt, and it all started when he entered the league at age 19.
The Kid had two terrific seasons before turning 22, but the most impressive was his 1991 campaign.
Junior wasn't yet the man who would hit 56 home runs in back-to-back seasons in 1997 and 1998, but he hit 60 home runs in his three years before turning 22.
After hitting 630 home runs during one of the most injury-plague careers in MLB history, it's hard to imagine that The Kid couldn't have broken the record. Unfortunately it was not meant to be.
Team: Oakland Athletics
DOB: July 28, 1949
Best Season: 39 games, 24-8, 312.0 IP, 1.82 ERA, 0.95 WHIP 301 K, (1971)
Notable Honors Before Age 22: All-Star; 1971 AL Cy Young; 1971 AL MVP
Vida Blue showed off just how good he was when he pitched a no-hitter in one of his 10 starts before the 1971 season, and from there, he dominated MLB hitters.
Despite getting knocked out in the second inning of the season opener in 1971, Blue would come back and lead the American League in ERA, while leading the majors in shutouts with eight.
Blue's six hits and 8.7 strikeouts per nine innings pitched helped his case for the Cy Young award and even the MVP in 1971.
Very few pitchers have ever won the MVP, but Blue did so at the age of 21. Needless to say, he had a pretty good start to his career.
Team: Cleveland Indians
DOB: November 3, 1918
Best Season: 43 games, 27-11, 320.1 IP, 2.61 ERA, 1.13 WHIP, 246 K (1940)
Notable Honors Before Age 22: Three-time All-Star
Bob Feller was one of the youngest pitchers in MLB history when he started with the Cleveland Indians, pitching as a junior in high school and striking out 17 batters in one game at the age of 17.
Feller was only getting warmed up, however, as in 1940, he led the MLB in starts, wins, innings pitched and strikeouts. He also led the AL in ERA, WHIP and strikeouts per walk.
While any of Feller's first five seasons in the big leagues could've made this list, it's his 1940 campaign that helps him grab the No. 4 ranking. He is now a member of the Hall of Fame as well.
Team: Los Angeles Angels of Anaheim
DOB: August 7, 1991
Best Season: 139 games, .326 BA, 30 HR, 83 RBI (2012)
Notable Honors Before Age 22: Two-time All-Star; Silver Slugger; 2012 AL Rookie of the Year
Mike Trout burst onto the scene last season, proving that he was the second-best player in the American League behind Triple Crown winner Miguel Cabrera.
At 20 years old, Trout showed off his ability as a five-tool player, leading the MLB in runs and stolen bases, while hitting 30 home runs and batting well over .320.
Trout was simply phenomenal, and his defense in the field was just as impressive as his batting, as he earned a reputation for taking back home runs.
Today is Trout's 22nd birthday, but before turning 22, he amassed 54 home runs and 167 RBI while batting .316. Not bad.
Team: New York Mets
DOB: June 10, 1982
Best Season: 35 games, 24-4, 276.2 IP, 1.53 ERA, 0.96 WHIP, 268 K (1985)
Notable Honors Before Age 22: Three-time All-Star; 1985 NL Cy Young
Dwight Gooden was freakishly good as a young pitcher, and 1985 was the best year of his career. That's right, his best year was at age 21.
Posting career-bests in wins, ERA, WHIP, innings pitched and strikeouts, Gooden dominated National League hitters, taking the bats out of their hands.
Gooden's pitches were working for him all season long, as he was nearly unhittable. It came as a surprise to no one when he won the Cy Young award and took home all 24 first-place votes. However, it did come as a surprise when that season turned out to be the best of his career.
Team: Seattle Mariners
DOB: July 27, 1975
Best Season: 146 games, .358 BA, 36 HR, 123 RBI (1996)
Notable Honors Before Age 22: Two-time All-Star; Silver Slugger
Despite everything that is going on with Alex Rodriguez these days, there's no denying that he stands alone as the best player at the age of 21 or younger in MLB history.
A-Rod's 1996 season was something special, as he led the MLB in batting average and doubles, and finished second in MVP voting, just three points behind Juan Gonzalez.
With his knack for hitting extra-base hits by finding the gaps and launching balls over the fence, Rodriguez quickly became one of the most feared hitters in the league, and it all started with his 1996 campaign.