When watching Carl Crawford, one trait jumps out above all others: Speed.

At 29, he is already one of the most prolific base stealers in major league history. His 409 career steals rank 37th of all time. After years of seeing the outfielder torture Red Sox catchers, little explanation is necessary to explain how much Crawford’s presence on the bases can transform a game.

The same can be said of the outfielder’s defense. In 2010, Crawford finally was awarded a Gold Glove for his tremendous work in left field, thus becoming the first American League left fielder in nearly 30 years to receive the honor. According to fangraphs.com, he has been — far and away — the best defensive player in the game for the last three years, having saved 52 runs compared to an average fielder at his position.

Speed is what has helped to make Crawford a star. Speed is the trait that made him a $20 million a year player for the Red Sox.

There is little doubt that the left fielder is growing as a hitter. He had career highs in homers (19) and OPS (.851) in 2010, but if you take away Crawford’s legs, he’s fighting players like Vladimir Guerrero and Hideki Matsui for a one-year deal in the $4 million to $5 million range.

But speed is a tool that starts to decline almost from the earliest days of a player’s major league career. And so it is fair to wonder: What will Crawford be over the life of his seven-year deal? How do players who are phenomenal base stealers at an early age perform as the odometer turns over from their 20s into their 30s?

That is the question the Sox confronted while trying to decide how far to go in their bidding. In addition to a thorough scouting analysis of Crawford, the team also asked analyst Bill James to study what could be expected of the outfielder before signing him to the biggest contract issued by this ownership regime.

A LIFELINE

The issue is one that James has examined at times for the better part of three decades. He said in an e-mail that his studies have shown that players with elite speed observe “dramatically different” aging patterns than other players.

That is in no small part, he said, because speed allows players to remain in the game. Players who are immobile lose their ability to play a position; consequently, they either have to hit like David Ortiz or disappear from the game.

“Speed is the lifeline. Remember those sooth sayers who would study your palm and tell you how long your life line is? Speed is the life line,” James wrote in an e-mail. “If a player has a longer life line, then he has more time to develop his other skills, and consequently he MAY develop his other skills. But that's not essentially why he lasts longer.”

Instead, James suggested, the ability to play defense is a key component to making a player valuable enough to keep a job. Speed impacts that equation directly.

“The relevant thing for speed and aging is defense,” James wrote. “As players age they are driven out of the game by two things: 1) the loss in batting skill, and 2) the loss in mobility, which forces them to play the defensive positions where offensive expectations are highest. 

“Suppose that you grade position players 1-10 based on a) hitting ability, and b) defensive mobility. Shortstops would often be 2-3 on hitting ability, but 7-8 on defensive mobility. As they age, they will lose defensive mobility, which will drive them out of the game because you can only play as a 2-3 hitter if your defensive mobility is very high. …

“An unnamed player might be a 6 on a hitting skill scale, and he might be a 6 on a mobility scale. As he ages he will slip in hitting skill and he will slip in mobility, and at some point, not very far in the future, this combination will drive him out of the game.

“But if the player is a 6 on a hitting scale and a 10 on a mobility scale, then he will (generally) last a great deal longer (than the 6-6), because even after he loses mobility, he is still mobile enough to play the defensive position that he has to play in order to stay in the game. That's the relevance of speed as a player ages. . .not stolen bases, but defensive mobility.”

Speed is an asset that does decline, however. There is little question that the Sox signed Crawford to be a starter for them for the next seven years. But what kind of player will he be over the life of his contract? How much of his current skill is he likely to sustain?

CRAWFORD IN CONTEXT — HOW BURNERS AGE

Crawford belongs on a short list of the top young base-stealers in baseball history. In an era when advance scouting is better than ever and when the slide step is a common tactic, he remains a constant source of agitation to pitchers and catchers.

Crawford is just the fourth player in major league history with 40 or more steals in seven seasons by his age 28 season. The other three are all (or should be) Hall of Famers: Rickey Henderson, Ty Cobb and Tim Raines.

But while Crawford offered the Rays an incredible threat on the bases, it remains to be seen whether he will be able to do the same with the Sox. Based on other players who were in his base stealing class in their early careers, it would appear likely that Crawford will not be as much of an impact player over the next seven years as he has been since becoming a big league regular in 2003.

Between the ages of 22 and 28, 192 players have produced a total of 378 seasons with 40 or more steals. Between the ages of 29 and 35, 89 players have produced a total of 172 seasons with 40 or more steals. (Most notable in that older demographic: The remarkable Lou Brock set the single-season steals record by swiping 118 bags at the age of 35.)

But what about players in Crawford’s class, who have established themselves among the top base-stealers of their generation? In order to get a slightly broader sample than Crawford and three Hall of Famers, it is worth looking at the class of players who stole 40 or more bases at least five times through their age 28 seasons.

It’s still a short list, with Crawford ranking as the 16th member of the group. The others: Henderson, Raines, Cobb, Delino DeShields, Juan Pierre, Vince Coleman, Julio Cruz, Cesar Cedeno, Bert Campaneris, Eddie Collins, Willie Wilson, Bobby Bonds, Max Carey, Clyde Milan and Sherry Magee.

Given that the Sox signed Crawford for his age 29-35 seasons, it is worth looking at what the other 15 players did from the start of their careers until they turned 28, and at what they did from the ages of 29-35.

For the most part, it’s not a terribly encouraging picture for the Sox. Arguably, just one player (Hall of Famer Max Carey) could claim to be as good during his age 29-35 seasons as he was before then.

Two of the players in the group improved what Mike Scioscia and the Angels like to call their batter’s box offense. Rickey Henderson improved his OPS+ by 3 percent, while Carey saw his OPS+ go up by 7 percent. Seven of the players had declines in their OPS+ of less than 10 percent; four fell by 10-20 percent; two declined by more than 20 percent.

Every one of the 15 players to have stolen 40 or more bases on five occasions by age 28 suffered a decline in playing time between the ages of 29-35. Three of them, in fact, were out of baseball by age 35. Seven saw their number of games played per year decline by less than 10 percent; five had their playing time decline by 10-20 percent; three saw their playing time plummet by more than 20 percent.

Most dramatically, the players in this group of young base stealers endured a dramatic decline in stolen bases. Juan Pierre, who is still active and played his age 32 season last year, has seen his steals drop by just 6 percent; Hall of Famer Max Carey had his steals decline by 7 percent. The other 13 players saw their swipes plummet by at least 30 percent per year between the ages of 29-35, as compared to their careers through age 28; eight of those saw their steals get cut by 50 percent or more.

If one merely goes by the history of players who ranked perennially among the top base stealing threats in their early careers, then the odds are that Crawford will not be as good a player for the next seven years as he has been to this point in his career. That dire assessment aside, it doesn’t mean he can’t still be a very good player.

REASON FOR OPTIMISM?

There is little doubt that Crawford is a vastly better hitter now than he was in his early career. Whereas there were times at the beginning of his career when he was all but impossible to walk, he has become a much more disciplined hitter, especially in the last two years. His pitches per plate appearance have gone up from 3.43 (through 2008) to 3.75 (in 2009-10). He has walked 97 times in the last two years, or almost as many free passes as he had taken in the previous three seasons (99).

Those numbers — as well as Crawford's growing power totals — are a direct reflection of an improved approach.

“As a hitter he covers so much more of the zone that he used to,” Sox general manager Theo Epstein said at the press conference to introduce Crawford. “That pitch away from him, he’s really comfortable hitting that ball to left field. At times, and I think this is what I think he’s going to work on this winter, really driving the ball the other way, not just slapping it the other way, but driving it.

“I do see sustainable power for him. Right field is a little bit deeper than parts of Tropicana. But when he hits them they’re no-doubters.

That improved plate discipline and knowledge of the strike zone likely plays into his career highs in slugging (.495) and OPS (.851) in 2010, when he played mostly as a 28-year-old. He does appear to be a hitter rising into his prime, and the Sox should benefit from his peak years as a hitter.

“That's what I would like to think," Crawford said in September. "I'm just starting to learn. My game was a lot of times hitting the ball in play and trying to run. Now I've got a feel on how to see the ball and I'm driving it better. I figure if I didn't have to steal 50 bases I would actually be a better hitter. I would have more strength in my legs to hit all year long, because that wears down over the course of the year."

Meanwhile, as James points out, even if Crawford's speed is in the decline phase, it will remain a weapon. Consider that if he suffers a 50 percent decline in steals, he’ll still be good for 25 stolen bases a year. Given that he has been the top defensive left fielder and one of the top overall defenders in baseball for several years running, even as his speed slips in his 30s, he should remain capable of impacting the game with his glove.

Presumably, he is a 9 or 10 on James’ hypothetical scale now. That being the case, there is substantial reason to think that even as Crawford loses a step or two in his 30s, he can still be a very good defensive outfielder into his 30s.

If his speed continues to allow Crawford to play defense at that level while sustaining a reduced-but-relevant base-stealing threat, then so long as Crawford can be at least a slightly above-average hitter, he could offer the Sox a solid return on their investment for the life of the contract. Players with such well-rounded skill sets are in short supply.

In the short term, the returns seem more certain. The Sox have acquired a 29-year-old who is one of the best and most dynamic all-around players in the game. But only time will tell how long that label will fit.