How these 37 and 38 year old players are performing so well in the IPL? A few days back it was Chris Gayle and now it is the turn of Shane Watson... In ODI or test cricket, these players would have been out of consideration for selection.
They just have to perform for 15-30 minutes for a good innings here. If there fitness levels are to be put to real test, that can be done with a test match. I think they will not even be able to qualify the medical fitness tests in such a scenario.