It is inconceivable to me that a team can disintegrate from what it was last year to what it is this year, especially since it has a better bench, and have the primary culprit be the aging of our core.
While it can be argued the team's impotence, apathy and general malaise can be attributed to a confluence of many of the choices in the poll, I believe that what set the tone for the season and has led to a domino effect in many ways is the fact that both Pierce and Garnett, hardliners for the players' union, believed/wanted there to be no season.
Believing/wanting there to be no season, neither Pierce nor Garnett prepared for it either mentally or physically. These guys are the two highest paid players and supposedly the spiritual/emotional leaders of this team.
Age? Ray came in in shape. Same for Rondo, Bass and just about everyone else. Even JO talked about intensely rigorous rehabbing and workouts?

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Before the season started, Doc talked about Garnett's minutes being 5 in 5 out. That was not just to manage Garnett's minutes as a strategy, but rather a compensation for Garnett being in terrible shape. He also talked about him playing center more. Again, not just a strategy dictated by personnel issues but also knowing that as a 5, Garnett would be less likely to be run into the ground as 4s are generally more mobile north to south than 5s.
Pierce not being ready for the season has showed itself in many ways. Inconsistency in both production and effort, injuries and attitude.
Again, while many factors have contributed to this terrible dropoff, and age is an easy scapegoat, I lay much of the blame at the feet of the Captain and Garnett. Both have made their money, and I don't think that either wanted to play this year. I guess that they are kinda getting their wish.