Romo is a different style of QB than Peyton Manning he relies on his athletic ability to slip defenders and get outside the pocket to extend and make plays. It becomes more difficult for those style of QB's to avoid defenders and get outside the pocket the older they get. Manning does his damage from within the pocket where as Romo isn't very good within the pocket. Pocket passers usually have more longevity than QB's who rely on their legs and athletic ability to get it done. McNabb's career started fizzing out at 33 because he couldn't move around as good as he did in his prime.