In professional sports, a player's career unfolds in glacially-slow increments. In much the same way it would be nearly impossible to pinpoint exactly when a geologic era ends and when another begins, it's -- albeit on a smaller scale -- difficult to measure when exactly a player enters his prime years.All you can do is look for indicators. For those of us searching for the start of LeBron James' career prime, Friday's convincing win over the Celtics might have given us the strongest piece of evidence yet that LeBron is ready to reach his zenith.Physically, LeBron has never not looked like a player in his prime years. He entered the league with an NBA body, NBA athletic talent and an incredibly-developed sense of how to handle NBA fame. Since then, he's only become bigger, more athletic and more savvy.The only ingredient that was really missing was his approach to the game. More specifically, his approach to defense.For his first five years in the league, LeBron did all of the right team-oriented things on the offensive end. He took over games when it was needed. He never stopped trusting his teammates, no matter how many three-balls Donyell Marshall bricked or how many passes smacked Drew Gooden's palms and landed out of bounds.He became a leader. He took Daniel Gibson under his wing. He respected what his coaches told him. He was and is a model teammate.But there is a certain amount of stubbornness associated with LeBron. Perhaps it's the product of having a superstar's ego. Perhaps he was simply more concerned with rounding out his offensive game. But LeBron's path to becoming a great individual defender seemed to meander far more than his path to stuffing the offensive stat sheet.LeBron could play great defense when he wanted, but too often in years past, he'd play the passing lanes looking for a steal, or play a sort of half-speed on defense to conserve energy for the next trip down to the offensive end.It was understandable to an extent when his teammates were Marshall, Gooden, Larry Hughes and Damon Jones, and LeBron was really the first, second and third options on offense -- reliable options, anyway. There are only so many ways one player can exert himself.But the rarefied air occupied by Michael Jordan? The multiple rings? Those weren't going to be within LeBron's grasp until he rounded out his game at the defensive end.Friday, in the fourth quarter, Mike Brown rotated LeBron onto Paul Pierce when other options proved insufficient. And for the first time on the big stage, we saw LeBron as a shutdown defender on an elite opponent.Pierce is Boston's go-to guy in the second half. He's the clutch-shot drainer. He's the guy who takes on the other team's best scorer in games of one-upsmanship.That's exactly into what Pierce was trying to convert Friday's game. A repeat of last spring's Game 7 between the Cavs and Celtics, when LeBron went off for 45 and Pierce for 41, and the Celtics edged the Cavs out of the playoffs en route to a 17th NBA title.Last season, LeBron might have obliged Pierce's challenge to go mano-a-mano. Last season, a game like this might have disintegrated into LeBron and Pierce exchanging shot after shot while the other eight guys on the floor ran the in-game equivalent of suicides, baseline to baseline.But this is a wiser LeBron, who realized that if Pierce caught fire from the floor, that would be Boston's best chance to get back into the game. With Pierce as his responsibility, LeBron made it a personal crusade to keep Pierce from finding clean looks at the basket.LeBron has made defensive highlights before. Many times it involves an emphatic block. He had one of those on Friday night, too. But his defensive highlight of the evening came on a Boston possession late in the third quarter when, without the benefit of a double-team or the edge of the court to act as another defender, he physically walled Pierce off from doing anything with the ball.Pierce was attempting to post LeBron, looking for any way to clear space for a shot. But LeBron stayed on him like a 260-pound wet t-shirt, blocking his attempts to go right and left, or pass the ball to a cutting teammate. Pierce was utterly walled off from any action moving toward the hoop. He had to relent, reset himself, and before he realized it, the shot clock grew late, and he was forced to fling a weak pass to Leon Powe, who tossed up an awkward miss.After that sequence, Pierce -- and by extension, the Celtics -- started to lose a bit of their composure. Pierce's final stat line: 11 points on 4-of-15 shooting, 1-of-5 from beyond the arc.When, exactly, LeBron learned that suffocating defense can intimidate opponents as much as an offensive barrage is not as important as the fact that he can now count it among his vast array of weapons. And that embraces the fact that he can use his superlative talent to play lockdown defense.LeBron's individual defense added momentum to the Cavs' team defense, which held Boston to 41.3 percent shooting from the floor. The Celtics' inability to solve Cleveland's defensive attack seemed to reveal something about their character. After attaining so much success in such a short period of time last year, it's apparent the Celtics are having a hard time dealing with their current struggles in a manner that doesn't involve sulking and pouting.Down by three and four possessions late in the fourth quarter, with Pierce and Kevin Garnett already sent to the bench muttering what we could assume were some not-nice things under their breath, Celtics coach Doc Rivers resorted to fouling Ben Wallace repeatedly. Wallace is a career 41.8 percent free throw shooter -- the worst of all time.Once or twice, it made sense to resort to a "Hack-a-Ben" strategy. But as Wallace kept splitting his pairs of free throws and the strategy failed to pull the Celtics any closer, Rivers continued to foul Wallace, in an apparent act of denial-slash-frustration.LeBron banked in a 40-foot shot on one of the Wallace fouls. It was a shot released in anticipation of the foul, and had LeBron released it a fraction of a second sooner, it might have resulted in a four-point play attempt, with three points going to LeBron and one possible point to Wallace.It would be foolish and incredibly premature to say the Celtics and Cavs are headed in opposite directions. But on Friday night, LeBron reached a new pinnacle while the Celtics -- and their collective composure -- continued to slide.We already knew LeBron could impose his will on a game from a pure athleticism and stat sheet-stuffing standpoint. But now, we are seeing a different type of LeBron emerge. One who excels at the psychological warfare of defensive basketball. The type of basketball that wins in May and June.If Friday's game is an teaser for LeBron's prime years, his prime will be a prime unlike any other.