McCown is a very good athlete. Who's to say, he could just be a late bloomer between the ears. One of McCown's former teammates said he is the most physically gifted "white boy" he'd ever seen. If you look at him, he looks quick, strong, fluid, accurate and confident. Add to that, an average arm and above average foot speed, you have a recipe for a modern QB.
I'm not concerned about his age either. As Adam pointed, he's younger than some that we're not worried about. Plus, he has less wear and tear than all of them. I'm somewhat sold on McCown now and the 14 mil(minimum) Cutler will command can buy us 3 to 5 starters on defense.