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Those guys in their 30s are working with a short window. It's not just that they will retire on average around 35-36, but their skills decline gradually before that happens.
If you're talking about HOF QBs like Brady and Brees, I think that statement is pretty false. Guys like Favre, Young, Elway and Moon have had top 5 (and better) years in their late 30s. Some HOFers fell apart (Montana missed a whole year due to an elbow injury and never put together a full year after that), but guys who stayed in 1 piece generally were highly effective from a fantasy perspective late in their careers.
What a QB loses in his 30s is arm strength. It's subtle at first but by mid-30s those who were without exceptional arm strength to begin with start to have difficulty making all the throws in tight spaces and the deep throws that they once could. They then have to adjust their game to account for that.Sure, there will be outliers who had such arm strength to begin with that they could get away with losing some of it and still be fairly OK. Notice that the names you mentioned were all rocket-armed in their prime? And some of those guys had stinker seasons surrounding the outlier stud year late in a career. I happen to believe P Manning will end up included in this elite-beyond-35 group because he's been that special, one of maybe the greatest 3-4 QBs in the history of the sport.
It's not much different than a strikeout pitcher in baseball. Their strikeout totals decline in their 30s and they are forced to use more guile and paint the corners better, because less and less they are able to just throw it by guys. By 35 they are in big trouble and finished if they haven't adjusted. Guys like Ryan and Clemens
seemed to still have it late, but that was because they started at a higher level and what they lost brought them down to what other guys start with (in Clemens case of course there's the whole steroid question too).
I'd be very worried about Brady, now 35. It's already becoming evident the arm strength is leaving him. His game is now mostly short stuff, and when he does go deep he's often under-throwing guys. It'll just get worse, and no matter how great he once was or how long he wants to keep playing, it'll be time for management to say move aside for the young guy. Don't be surprised, he may only have two years left. Brees isn't there yet (just 33) to the point the strength loss is evident, but it will become more so in the next few years. The Saints are balking at paying the man $25 mil per over the next 6 years. Why would they do that if he'll still be the same guy 4-5 years from now? Because they know he won't be. Time gets them all.
Most fantasy guys are a year too late realizing a player is losing it, and also a year too late realizing a young WR or QB is going to be elite. By ranking Luck and Griffin where I believe most will be ranking them once they've figured things out, I'm avoiding being behind the curve and kicking myself in two years for owning a declining Brees or Brady when Luck / RGIII are atop everyone's rankings. I can't in good conscience simply create redraft rankings and then move guys up or down 1-2 spots here and there to call them dynasty rankings. That's what most people do and I think it's far too short-sighted.