Most young players cannot play NBA-caliber defense. Since they began learning the game in the AAU circuit as teenagers, they have had a massive size and athletic advantage on their peers, letting them get away with bad habits defensively.
Even the ones who weren't offensive stars in college and understand the importance of consistent defensive intensity are mostly clueless on help defense. There's a reason most coaches dislike playing rookies: their heads are spinning way too fast to understand pick-and-roll coverages and team defensive concepts.
As a result, the default assumption is that every young player will get better defensively. But understanding how to play NBA-caliber defense is only a necessary, not a sufficient, condition to being an NBA-caliber defender. If a player lacks the prototypical size or athleticism for their position, they are never going to become good at defense. They might become like Steve Nash or Dirk Nowitzki, smart veterans who know their limitations and can make crafty defensive plays. But even with Nash's ability to take help-side charges and Dirk's ball-swipe move in the post, both need to play with versatile defenders so they can be hid defensively.
Individual defense comes down to two main factors: foot-speed and wingspan. Elite NBA defenders all have the same profiles: great athletes with long arms. Rajon Rondo, the first-team All-Defense point guard, is a perfect example: an athletic 6'1 guard with a 6'9 wingspan.
Over the years, Shane Battier has been widely heralded for his basketball IQ and almost encyclopedic knowledge of his opponents' moves. However, physical ability has been just as important for the two-time All-Defensive team selection. Battier is a 6'8 small forward with a 6'11 wingspan that allows him to suffocate shorter offensive players on the perimeter, and as he's lost foot-speed as he aged, his defensive rating in Houston suffered, going from 102 in 2008 to 108 in 2011.
In a game of one-on-one between two equally skilled basketball players, the longer and faster one can take the ball to the front of the rim and score and there's very little the defender can do to stop him. In the NBA, where the vast majority of players can knock down an open jumper, being unable to contest a player's release point is essentially giving away a basket.
If your hands cannot reach as high as your opponents, they might as well be shooting over a chair. An explosive leaper can make up for this by crowding a shooter and preventing them from getting air-space, but such tight defense is easily susceptible to pump-fakes. In contrast, a longer player can prevent a good look at the basket simply by keeping his hands straight up in the air.
There are outliers like Chuck Hayes, a 6'7 bowling ball of a power forward who uses a low center of gravity to stymie low-post scorers despite having only a 6'10 wingspan. However, his lack of prototypical size or athleticism prevents him from protecting the rim, as he's averaged only 0.8 blocks per 36 minutes over his career.
Derrick Rose won the MVP award last year because no point guards had the size and speed to stay in front of him. The only player who could was LeBron James, a 6'9 forward with a 7'0 wingspan who is just as fast as Rose. In the fourth quarters of the Eastern Conference Finals, Rose's drives were stymied by LeBron, forcing him to take step-back jumpers over a leaping 6'9 defender.
And no matter how many years an unathletic defender plays in the NBA, their arms won't get any longer and their feet won't get any faster. That type of defensive ability can be measured in the NBA's younger players, which gives us a good indication of which ones will become elite defenders and which ones will not.
John Wall, a 6'4 point guard with a 6'9 wingspan and a 39' max vertical, projects to be an elite defender at both back-court positions. Tyreke Evans, a 6'6 guard with a 6'11 wingspan and an explosive first step, could one day defend all three perimeter positions. Stephen Curry, an unathletic 6'3 guard with a 6'3 wingspan, couldn't.
None of the three are considered good defenders, primarily because none play for a coach with a strict defensive system who commands respect. But if Wall or Evans played for Tom Thibodeau in Chicago or Gregg Popovich in San Antonio, they would be benched for half-hearted defense and would quickly learn what to do to stay on the floor. Curry, in contrast, would still be a defensive liability no matter what system he's in, like Kyle Korver in Chicago or Matt Bonner in San Antonio.
These types of athletic deficiencies are often hand-waved away when discussing the potential of young players. In an article calling Blake Griffin the next KG, ESPN's Tom Haberstroh compared their offensive statistics with little mention of their defensive measurements. But there's no chance Griffin could ever match KG's defensive impact: Griffin is a wide-bodied 6'9 forward with a relatively normal 6'11 wingspan; Garnett is a long-limbed 6'11 forward whose wingspan, while never officially measured, is routinely described as "freakish".
Similarly, Kevin Love's poor individual defense is usually written off as a function of his age. But the 6'9 forward has a wingspan of only 6'11; in comparison, LaMarcus Aldridge has a 7'5 wingspan. There's no scenario, barring a career-ending injury for Aldridge, where Love becomes a better defensive player in the next decade.
In evaluating young players, it's more important to know who can play defense than who wants to. A human being can change his attitude a lot easier than his wingspan.