Right ...
Because when the Celtics won the title in 2008, none of KG, Pierce, and Allen was under 30 (respective ages of 32, 31, and 32).
In contrast, LBJ, Wade, and Bosh RIGHT NOW are 29, 32, and 30.
So while I'll be more than happy to agree with you that coaching KG, Pierce, and Allen is worth putting up there as saying that you had talent to work with, being able to win with your core players over 30 in what is a young man's league is saying something.
2008: Path to the final included 7 game series with Atlanta (with young Al Horford and Joe Johnson on the squad) and Cleveland (with LBJ), beating a Detroit team at the end of a 6 year run in the East Finals, and then beating the Lakers in the Finals.
2009: Fended off a young Bulls team in 7 before losing to Orlando in 7 (who went on to the Finals) - KG was hurt
2010: Lost to the Lakers in 7 in the Finals (Kendrick Perkins blew out his knee in Game 6 of the Finals); beat Wade, LBJ, and Orlando (Howard) to win the East.
2011: Lost to Miami in 5 games after Rondo's elbow was dislocated by Wade in what could have been argued to have been a dirty play
2012: In what was really the last stand for the team, they lost in the East Finals in 7 games to Miami in a series that could have gone either way.
When LBJ, Wade, and Bosh are in their mid-30s and still right in the midst of winning the East, then we'll talk about how great Spoelstra is as a coach. Right now, it looks to me as if they have been running on fumes and in desperate need of reinforcements. They are about to enter a series against a number of players that have caused them problems in the past and has more depth than them. They are about to be in a massive dogfight. And for what it is worth, I'll be shocked if Wade is still playing in a starting role (if at all) in an Eastern Conference Finals at the age of 35.