Maybe so. I used to wonder how much better we'd have been had we aquired a 23~24 year old Matt Schaub. Sure, he wasn't a better athlete at 23 as opposed to 26, but 23 year old bodies heal better, faster.
26, 27, 28... could be some of the best years for a QB. Helps if he's not "adjusting" & learning from his mistakes.
It's rare enough to find a diamond in the rough undrafted (Hoyer), or in the 4th round (Savage, a 5th really), or in the third round (Mallett). Even rarer to find that undrafted, 3rd, or 4th round diamond past his 26th birthday.