I agree that the article isn't bad. It is a question more than an answer. What is to blame for this statistical decline?
I don't believe age affects decision-making to any great degree. You can point to there being missed throws to open men, or pushing throws to Gronk since his return in triple coverage or in ill-advised passes. Or throws off the mark. If Brady is entirely to blame, we have watched a series of games in which he (1) has been clobbered by bad blocking, (2) has consistently not had open men to throw to in passing plays or (3) is putting the ball to where it should be, only to find a receiver is not. These concerns may explain the current problem.
Brady is an intelligent QB, and a competitor. All of these problems, when arising on a regular basis, will encourage him and virtually any other QB, to press, push the ball and rush passes. The Pats offense has worked on precise timing and route running, in which Brady's job is to scan the field quickly and put the ball to the open man at the time and place it is designed to get there. He has been very accurate, and phenomenal in that system. Now, it is like owning a Ferrari and slamming your foot on the accelerator only to have the engine knock and sputter its way into oncoming traffic. Do you drive that car the same way? No.
I don't believe Brady is a 'system' QB, that his history and statistics would be the same as any QB plugged into his job, but he does work within a complex system. When nothing is working, he does not have 10 seconds to wait until it does. He is getting sacked too much for holding the ball waiting for something to develop, is pushing the ball to guys who have shown to be somewhat clutch (JE and Gronk), and is rushing passes because of pressure (real or perceived). That is a mental issue far more than a physical one, and a fixable problem if Brady starts to see the system a little more healthy than it has been. QBs like Elway and Favre both had statistically great years far later than Brady, and both relied on a broader physical skill set than Brady possesses (ie., their legs as well as their arms), skills like scrambling that do degrade quickly with time.