As one of the handful of teams to win repeat championships, these Miami Heat have cemented their place in NBA history. They’ve easily been the NBA’s dominant team since their Big Three formed before the 2010-11 season, with three consecutive NBA Finals appearances, two straight titles, and more regular-season wins and far more playoff wins than any other team.

After surviving three elimination games in this year's postseason and coming from behind in five playoff series over two years, Miami’s championship heart and resolve are beyond dispute. But the Heat also showed far more vulnerability in these playoffs than was expected after a dominant regular season. Now, NBA history suggests this Heat group will be hard-pressed to keep its window open.

First, it’s hard for anyone to keep a run like this going, given how physically and mentally grueling it is to survive the NBA marathon for multiple seasons. The only teams to ever make the Finals for more than three consecutive seasons are three of the most legendary in NBA history: the Bill Russell-led Boston Celtics (who made a miraculous 10 in a row from 1957 to '66), and the ’80s Los Angeles Lakers (1982-85) and Celtics (1984-87).

In their third straight season of playing well into June, the Heat often struggled to summon the maximum energy required to execute their blitzing defensive scheme. Now they're faced with scaling the mountain a fourth time.

That tidbit certainly isn’t a disqualifying factor in and of itself, but it becomes more troubling when coupled with another fact that became more apparent as the playoffs unfolded: These Heat have gotten old very quickly, with no clear way to reverse the trend.

"Effective age" measures a team’s average age by including minutes played with the average of the players on a roster (so 40-year-old Juwan Howard doesn’t artificially inflate Miami’s average, considering he wasn’t actually playing).

Using regular-season minutes, Miami’s effective age jumped all the way to 30.3 in 2012-13 (from 28.3 in 2011-12). That makes the Heat the sixth-oldest champion in NBA history (see chart).

(Note: These effective age numbers were calculated using Basketball-Reference, which lists a player’s age on Feb. 1 of a given season.)

The ’98 Bulls and ’69 Celtics -- the ends of the Jordan and Russell eras, respectively -- were famously on their last legs, while the ’11 Mavs were a one-hit wonder who enjoyed a charmed run and the ’07 Spurs needed a full overhaul of role players to return to true contention.

This year’s Spurs, by contrast, were the younger team in the Finals, with an effective age of 28.6 for the season (a fairly average number for a champion in the modern era). Despite the elevated age of its big three, San Antonio has made sure to fill out its rotation with sub-30-year-olds such as Kawhi Leonard (21), Danny Green (25), Tiago Splitter (28) and Gary Neal (28).

The Heat, though, have done very little to replenish their supporting cast with youth, opting for 30-something role players outside of Mario Chalmers (27) and Norris Cole (24). While there are various player options and the potential for retirements and the use of its amnesty provision, Miami is likely committed to Ray Allen (37), Shane Battier (34) and Rashard Lewis (33) through 2014, and Mike Miller (33), Udonis Haslem (33) and Joel Anthony (30) through 2015. Another player who seems vital to re-sign, Chris Andersen, is 34.