The NFL world was left with its proverbial jaw on the floor when Indianapolis Colts quarterback Andrew Luck retired at just 29 years old, having thrown for 39 touchdowns last season. Luck's mental fatigue and physical deficiencies have forced him to hang up his cleats earlier than anyone expected, and he now sits high on a list of players who were at the top of their game, but chose to call it quits.
8. Robert Smith
Smith was a two-time Pro Bowl selection at running back over eight seasons with the Minnesota Vikings. In 2000, he had his best season yet, rushing for an NFC-best 1,521 yards and seven touchdowns. Despite his individual success, Smith chose to retire at the age of 28, citing his concerns over suffering a debilitating injury like so many others at his position. Smith, a former Ohio State star, has gone on to a successful career as a college football analyst for ESPN and FS1.
7. Tony Boselli
Selected second overall in the 1995 draft (thanks to Cincinnati drafting all-time bust Ki-Jana Carter at No. 1), Boselli was the first star the expansion Jacksonville Jaguars ever had, making five Pro Bowls, earning three first-team All-Pros, and making the 1990s All-Decade team as a left tackle. A nagging shoulder injury diminished his effectiveness, but he was still a solid tackle in his later years. Rather than join another expansion team in the Houston Texans, Boselli instead retired after seven years in Jacksonville.
6. Andrew Luck
There's no other way to put it: Luck's retirement is stunning. The injury history was long and concerning, yes, but no one could have imagined a 29-year-old on a Super Bowl-caliber team coming off the best season of his career in which he threw for 4,953 yards and 39 touchdowns would hang it up two weeks before the season started. With his multi-millions in the bank and a Stanford education, Luck is almost guaranteed to succeed at whatever venture he puts his mind to next.
5. Patrick Willis
When Willis led the league with 174 tackles as a rookie, you knew he was going to be something special. The former San Francisco 49ers defensive superstar was the best linebacker in the NFC for most of his career, making seven Pro Bowls and six All-Pro teams while leading the league in tackles twice. His sudden retirement after 2014 at the age of 29 caught everyone off guard and left a hole in the middle of San Francisco's defense they have yet to fill.
4. Earl Campbell
At least top three all time in terms of pure power running backs, Campbell was the unquestioned running back of the late 1970s, powering over everyone who foolishly attempted to tackle him en route to three consecutive All-Pro teams and rushing titles. A Hall of Famer known for his brutality as a runner and dominance, the Houston Oilers legend's style of play caught up to him, and forced him out of the game at age 30, despite having a lot left in the tank, in theory.
3. Calvin Johnson
Megatron was the best receiver in the league for the first part of the 2010s, as he and Matthew Stafford put up numbers together that were just remarkable. The 6-5, 237 pound speed demon still owns arguably the greatest single season a receiver has ever produced, when he caught 122 passes for a still-record 1,964 yards. Ultimately, Johnson's frustration with the Lions organization got the best of him, and he retired at age 30 after just nine seasons.
2. Barry Sanders
Losing in Detroit certainly takes its toll on you. Despite playing with quarterbacks like Rodney Peete, Scott Mitchell, and Charlie Batch for his whole career and having Wayne Fontes as his coach for most of it, Sanders remained the most electric runner the league has ever seen. He made a Pro Bowl in all of his 10 seasons, was a first-team All-Pro six times, won four rushing titles, and ran for 2,000 yards in 1997. However, after falling out of love with the game, Sanders retired after a 1,491-yard season in 1998.
1. Jim Brown
There was never a more dominant player in the history of football. Brown was the Cleveland Browns running back for nine seasons, and led the league in rushing yards in eight of them and rushing touchdowns in five of them. A first-team All-Pro in all but one season, in which he was second-team, Brown retired at the age of 30 after just nine years in Cleveland. Brown is likely the greatest running back who ever lived, but that title would have been even further cemented if he'd played just a bit longer.