History says Bryce, Manny will ...

Bryce Harper and Manny Machado were picked first and third overall in the 2010 Draft. Both debuted in the Majors just two years later, Harper as a teenager and Machado shortly after his 20th birthday. Both quickly became stars, and now both have reached free agency in perfect position to land massive contracts.
Not only are Harper and Machado talented players with strong track records, but they just finished their age-25 seasons. That puts them firmly in their primes and, in theory, much further away from declining than the typical player, who would first hit free agency around age 30.
• Sizing up suitors for Manny, Bryce (or both)
Both players also certainly have their detractors, whether due to fluctuations in their performance, flaws in their all-around skill set or perceived issues with how they play the game and conduct themselves. In the big picture, however, precedent strongly suggests that hitters who are this productive this early -- by forcing their way into MLB at a young age, and then performing at a high level -- tend to stay productive.
According to FanGraphs, Harper and Machado both have roughly 30 Wins Above Replacement (WAR) in their careers. (So does Mookie Betts, but he isn't scheduled to reach free agency for two more seasons.) That's not a perfect measure, of course, but it's a good overall approximation of how they compare to others, both currently and throughout baseball history.

To give an idea of what teams might expect from Harper and Machado over the next decade -- each easily could land a contract of that length -- MLB.com went looking for position players with comparable early success. That meant collecting a group of 43 players since World War II who also generated between 20-40 WAR through their age-25 seasons and have seen at least 10 years pass since then.
Excluded were a select few superstars who were even better as 25-and-under players: Mike Trout (54.9 WAR), Mickey Mantle (52.5) and Alex Rodriguez (42.8). Also left out were a handful who remain too young to have completed that ensuing 10-year period.
Over the following three years (age 26-28), the remaining group of 43 had an average WAR of 16.0, with a median of 17.2. Over five years, the group averaged 25.3 WAR, with a median of 25.5. And over 10 years (through age 35), it averaged 40.1 WAR, with a median of 42.1. A season worth 4-5 WAR is generally considered All-Star caliber, so that gives you an idea of how good these players remained over time.
Here is a breakdown of that group, with each player listed with his WAR from his age-26 through age-35 seasons.
Cream of the crop
Willie Mays (92.2 WAR), Barry Bonds (79.1), Hank Aaron (75.7)

Mays and Aaron are inner-circle Hall of Famers. Bonds has the numbers to join, or even lead, that class. All three continued to play at a high level even after age 35, with Bonds actually setting the single-season home run record at 36, when he won the first of four straight National League Most Valuable Player Awards.

All-time greats
Rickey Henderson (62.1 WAR), Carl Yastrzemski (58.2), Frank Robinson (57.9), Eddie Mathews (56.7), George Brett (53.7), Albert Pujols (51.8), Cal Ripken Jr. (50.9)

All six of the retired players are in the Hall of Fame, and Pujols will one day join them all in Cooperstown. All except Mathews won at least one MVP Award during that 10-year stretch.
Henderson remained the game's top leadoff hitter and stolen-base threat, while Yastrzemski and Robinson both hit their way to Triple Crowns. Brett batted better than .300 seven times. Ripken, of course, played every day. And Pujols kept his title as MLB's best hitter for the next five seasons before his performance began to dwindle with the Angels.
Superstars
Miguel Cabrera (48.5 WAR), Scott Rolen (47.6), Eddie Murray (47.4), Reggie Jackson (46.7), Al Kaline (46.3), Gary Carter (45.0), Ron Santo (44.9), Adrián Beltré (44.9), Ivan Rodriguez (43.1), Bobby Grich (42.7), Frank Thomas (42.2), Robin Yount (42.1), Ken Griffey Jr. (41.9), Johnny Bench (39.2), Roberto Alomar (37.3), Tim Raines (36.8)

Not all of these players remained stars throughout the entire next 10 years, but 12 of the 16 produced enough to punch their tickets to the Hall of Fame, with Cabrera and Beltre looking like good bets to join them once their careers are over. That leaves only Rolen and Grich, both of whom are severely underrated (and Rolen having just gone through his first year on the Hall of Fame ballot).
Cabrera and Yount both won multiple MVP Awards during this time, with Rodriguez, Thomas and Griffey each taking one. Other highlights include Cabrera's 2012 American League Triple Crown, Jackson leading the AL in total homers over that span, Rodriguez and Alomar's seven Gold Gloves apiece and Griffey's three consecutive home run titles.

Productive, for a while
Bobby Bonds (36.5 WAR), Joe Torre (35.6), Dick Allen (33.1), Willie Randolph (31.8), Ted Simmons (30.5), Andruw Jones (30.3), Orlando Cepeda (26.4), David Wright (24.1), José Reyes (22.5), Nomar Garciaparra (21.5), Carl Crawford (20.8), Jose Canseco (20.8), Vada Pinson (17.5), Cesar Cedeno (17.4), Jim Fregosi (17.3)

Each of these 15 players had good seasons in this 10-year window, but injuries and/or age typically caught up to them sooner or later. Group members averaged nearly 4 WAR per season from ages 26-30 but only about 1.5 WAR per season from 31-35.
Wright, whose career just came to a premature end, is a representative example. The Mets third baseman was one of the best players in baseball over his first five seasons (28.1 WAR). That productivity waned a bit over the next five (20.8), but Wright's numbers remained stellar, and he made four All-Star teams. Then, after Wright's age-31 season, physical issues held him to just 77 more games.
There are still plenty of accomplishments to go around. Torre, Allen and Cepeda all won MVP Awards. Jones won five more Gold Glove Awards in center field and hit 51 homers at age 28. Garciaparra and Reyes collected batting titles. But, at least to this point, only Cepeda has gotten to Cooperstown -- and that was via the Veterans Committee.

Worst-case scenarios
Jim Ray Hart (4.0 WAR), Grady Sizemore (1.4)

Sizemore represents the nightmare lurking in the minds of any general manager thinking of committing hundreds of millions of dollars to Harper or Machado. The five-tool center fielder racked up 27.2 WAR over his first four full seasons, from ages 22-25 (2005-08), ranking fourth in the Majors, behind only Pujols, Chase Utley and Alex Rodriguez.
Then, at age 26, injuries struck, helping cause a dip in performance in 2009. Sizemore never really recovered, collecting barely 1,000 more at-bats between 2010-15, while enduring numerous surgeries.
Ongoing cases
The most recent history, while still incomplete, might give teams a bit more hesitation. Hanley Ramirez has 17.7 WAR over nine up-and-down seasons since age 26, having been released by Boston in the middle of this past season. Evan Longoria remained a solid contributor from ages 26-30 (19.3 WAR), but he has declined since and posted a well-below-average 85 wRC+ at the plate in 2018. Ryan Zimmerman has averaged 112 games and 1.8 WAR over eight injury-plagued seasons.
Giancarlo Stanton has authored two good-not-great years sandwiched around his 2017 NL MVP Award-winning campaign. Kris Bryant only just played his age-26 season, but he dealt with shoulder issues and saw his production take a big step back.
Still, the book remains open on those six active players, especially Stanton and Bryant. None was as good through age 25 as Harper or Machado, and the larger sample of results over the past several decades is strong.
As interested teams ponder their bids for this offseason's two elite free agents, they know full well that baseball offers no guarantees, especially regarding players' health. But as history illustrates, players who perform at an elite level early in their careers are good bets for future success.