You are falling into the trap of comparing today to the past. The average QB from as recently as ten years ago was in far worse physical shape than the players at the same position today. Brees has been at the cutting edge of this trend. Today Brees at 37 is in better physical condition than the vast majority of players at thirty ten years ago. At 40 he will still mostly likely be better fit than players ten years younger ten years earlier.

Brees has repeatedly said he plans to still be playing at a high level well into his forties, and there is nothing that he has said or done that would make me not believe in that prediction.