This is a link, but it deserves your attention.
Advanced NFL Stats did a piece investigating how QBs age, and the results are similar to something I found a while back.
The bottom line is that very successful quarterbacks like Manning aren't going to become bad slowly. All of sudden one year, they'll have significant drop-off in performance. If they were 26 and had the same kind of season or had a similar injury, they'd no doubt be back at camp the following July. But at 36, that job in the broadcast booth will seem quite enticing. Successful, established QBs will generally continue to be successful until one day they're not. We won't see it coming. But of course, everyone will pretend they did.
In my Old Manning piece from 2009, I wrote:
In terms of playing time, we won't see the end coming. The greats kept throwing until the end. Once they couldn't be the man, they walked away. The exceptions were Warren Moon, who started in the NFL much later than other great QBs, and Johnny Unitas.
It's a good piece, and worth your time.