I disagree that either Brady or Manning is past their prime. Yeah, they aren't what they were five years ago, but they are still some of the best the NFL has. Both had exceptional seasons offensively, and had one bad game ruin what could have easily been a Super Bowl win for either one. Nobody has the experience and talent right now in the NFL that these two have, and they could easily have another 1-2 more solid seasons before it would be time to throw in the towel.
I may hate Peyton Manning, but I still have mad respect for him and his skill, and his class.