Andrew Luck shocked the sports world Saturday when he announced his retirement from the NFL.

Luck spent his entire six-year career with the Indianapolis Colts after they took him in the first round of the NFL draft in 2012. The four-time Pro Bowler was also recently named the NFL's Comeback Player of the Year in 2018 after missing the 2017 season while sidelined with a shoulder injury.

But it had not been smooth sailing for the quarterback since, who said he was consistently struggling with rehabbing his shoulder. That struggle lead to what Luck called "the hardest decision of my life."

"I've been stuck in this process," Luck said after the Colts preseason game on Saturday. "I haven't been able to live the life I want to live. It's taken the joy out of this game ... the only way forward for me is to remove myself from football. This is not an easy decision. It's the hardest decision of my life. But it is the right decision for me.''

The stunning retirement of Luck, who turns 30 in September, got us thinking about other shocking early retirements.

Calvin Johnson

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In 2016, at age 30, the wide receiver and six-time Pro Bowler announced he would be stepping away from the game.

Nicknamed "Megatron" and considered one of the greatest wide receivers of all time, Johnson spent his entire NFL career with the Detroit Lions, who drafted him second overall in 2007.

In his final season in the league, Johnson racked up 1,214 yards and nine touchdowns, but that wasn't before he broke Jerry Rice's single-season record in total yards in 2012, finishing with 1,964 yards. Johnson later opened up to ESPN about why he stepped away from the game, citing his history of concussions and injuries.

Magic Johnson

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On Nov. 7, 1991, the L.A. Lakers point guard unexpectedly announced he had tested positive for HIV and was retiring from the NBA after 13 seasons.

Johnson, who is now 60 years old, retired as the NBA's all-time leader in assists. He also won the NBA MVP three times, as well as Finals MVP and was named to the NBA All-Star team 11 times during his professional career.

He did come back to play in the 1992 NBA All-Star Game at the demand of the fans, and was part of the 1992 U.S. Men's Basketball team -- aka the "Dream Team" -- that won the gold medal at the Summer Olympics. Two years later, Johnson reunited with the Lakers as an interim coach near the end of the 1993-94 season. After that, he returned for one final stint as a player on Jan. 29, 1996 (averaging 14.6 points, 6.9 assists and 5.7 rebounds over 32 games) before retiring for good during the summer of 1996.

But here was one more shocking moment Johnson had up his sleeve. After being named the Lakers president of basketball operations in 2017 and signing superstar LeBron James to a four-year contract in 2018, he abruptly resigned from the team in April 2019.

Tracy Austin

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Unfortunately, Austin had no control over why she had to ultimately leave tennis. The former World No. 1 tennis player won three Grand Slam titles, including the U.S. Open in 1979 at the age of 16, making her the youngest female or male to win the title. She also won the U.S. Open again in 1981, as well as the mixed doubles at Wimbledon in 1980.

But lower back began to limit her in 1981, and she was later diagnosed with sciatica that caused her to miss months at a time in the years following.

In the midst of her comeback, at the age of 26, on Aug. 3, 1989 in New Jersey, Austin was hit by a van while making a turn at an intersection. The injuries from the accident, coupled with her chronic back issue, caused her to have to officially step away from the game of tennis in July 1994 after numerous comeback attempts. Despite the short career, she was inducted into the International Tennis Hall of Fame in 1992 at the age of 29.

Gale Sayers

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The halfback spent just seven years -- 1965 to 1971 -- in the NFL, but was actually limited to playing five seasons due to injuries to both knees, among other minor injuries. But those five seasons with the Chicago Bears were certainly ones to write home about.

In the mere 68 games he appeared during his professional football career, he notched a total of 4,956 yards and 39 touchdowns. The four-time Pro Bowler was also the 1965 NFL Rookie of the Year and the 1969 NFL Comeback Player of the year, and led the league in rushing yards in 1966 and 1969. In 1972, Sayers retired at the age of 29 due to limitations from those injuries. Despite his short career, he was inducted into the Pro Football Hall of Fame in 1977.

And here are some more notable athletes who retired early from the last time we did this (shout-out Page 2!):

Jim Brown

After nine seasons with the Browns, the great fullback retired in 1965 at age 30. In his final season he rushed for 1,544 yards on 289 carries, scored 21 TDs, and was named MVP in leading the Browns to the NFL title game. He had led the NFL in rushing yards in eight of his nine seasons and left the game as its all-time leading rusher.

Brown announced his retirement in a mid-July news conference in London, near where "The Dirty Dozen" was being filmed. He still had one year to go on a contract that paid him $60,000 a season.

Barry Sanders

Sanders was only 31 and within 1,500 yards of breaking Walter Payton's career rushing record when he hung up his cleats just before training camp began in 1999. "The reason I am retiring is very simple," Sanders said in a statement. "My desire to exit the game is greater than my desire to remain in it."

In 1998, Sanders, a Pro Bowl selection all 10 of his NFL seasons, had run for 1,491 yards on 343 carries. The Lions made the playoffs in five of his 10 years, but had won just one playoff game.

Sandy Koufax

Koufax retired at age 30, directly citing a fear of permanent injury to his arthritic left elbow. Koufax, then making $125,000 a season, had just won his second straight Cy Young Award after going 27-9 with a 1.73 ERA in 1966. Don Drysdale was surprised by the November announcement, and although nobody questioned that Koufax was indeed suffering severe arm problems, at least one, AL president Joe Cronin, wished he had soldiered on. "He's too young to retire," Cronin said. "I'm sure from what he did last season he could go on pitching for a good many more seasons."

Isiah Thomas

In April 1994, Thomas, 32, ripped his Achilles tendon. Less than a month later, he announced his retirement, but said the injury was not a factor in leaving the court after 13 seasons in Detroit. "The Achilles injury wasn't the problem," Thomas said. "I'll be walking next week. The thing that makes me good is the energy and intensity I can bring to the game every night. I don't have that type of energy anymore. I don't have that rah, rah rah anymore. There's just no more energy left in my body."

In his final season with the Pistons, Thomas played 59 games, and battled other injuries, including a broken rib and a hyperextended knee. Still, managed to average 14.8 points and 6.9 assists per game. The Pistons finished the 1993-94 campaign with a 20-62 record, one of the worst in the NBA.

Björn Borg

After winning five straight Wimbledon championships between 1976 and 1980, Borg, who'd prevailed over great rivals like John McEnroe and Jimmy Connors, announced he was quitting the game on Jan. 22, 1983. Borg, who had turned pro at 16, was only 26.

"I know I could play another five years," Borg told Neil Amdur of the New York Times. "So to make this step, I wanted to be 101% sure before I decided. To retire at 26, that's very, very young. Just telling the simple truth that I don't enjoy it, I'm not motivated and I need to try other things. To take that step is difficult for a lot of people."

Rocky Marciano

After going 49-0 as a pro, Marciano, the world heavyweight champ, announced his retirement in April 1956 after a year of speculation. The Brockton Blockbuster was only 31.

"I didn't get hurt physically while fighting," Marciano said. "My physical condition has nothing to do with my retirement. ... My lonesome family convinced me that I should quit while I'm still in good shape."

Marciano vowed not to repeat the mistakes of past champs, like Joe Louis, who had attempted comebacks after announcing their retirements. But Archie Moore, the light-heavyweight champ and Marciano's last victim, didn't believe it would stick. "Marciano won't quit, because he loves the jingle of the American dollar too much," Moore said.

Moore, who himself first announced his retirement in 1941, 23 years before he finally took off his gloves for good, was wrong. Marciano didn't come back.