I think it is a combination of injuries (age plays a role in the injuries, but it is not the direct reason for the decline IMO), and attitude.
This team still is good enough to win a championship. However, those two problems all came together at the same time in the last couple of months to knock this team down a couple notches. Now that they have recovered a bit from the injuries, and they seem to have regained a bit of focus, they are getting back on track. However, those are the two things that could derail them in the playoffs. If they are not healthy, they simply don't have a chance, and if they do not have complete focus, they don't have a chance either.
This team is not too old to win. However, because of their age, they have many more hurdles to get past if they do want to win.