It isnt like Manning's last good year was 2007. Also people talk of QB'S like Elway (who was my favorite growing up) and Favre etc. Many of these guys are or were known for things such as arm strength or feet etc which would fade with age.
PM isnt known for either. His passes in fact rarely look very pretty as far as a tight spiral. What he does have is accuracy and an incredible intelligence of running the offense.
I think and hope he will be able to continue these things in the Mile High City.