I still think Brady's struggles are more a mixture of poor line play, inexperience with his receivers and their struggles with the offense, and him trying to do too much at times. I don't buy he is getting old or declining.
There might be a slight decline, but he is not showing things that come with age for a QB. He still has a quick release when he is confident in his receiver. He still throws the ball really deep and usually overthrows his receiver if he is off target (he would underthrowing more if it was because he is getting old). He still can move around the pocket to avoid the rush (at least when the defender doesn't have a clean shot at him unblocked like has been happening the last few games).
When a QB gets old, his release gets slower, his deep balls get shallower, and his mobility slows down. None of that is Brady's problem this year.