Given that the average age of the NFL draftee is about 23 years of age (22 for higher drafted picks, I'm guessing Day 1 and Day 2 picks, which most of the guys you mentioned probably were), your results certainly prove the well known adage surrounding 30 years of age. Their max productivity caps somewhere around 27-28, remains acceptable for another 2 years and then they are in free fall. There's always the exceptions like Frank Gore, but by and large, 29 or 30 is the cutoff age.
The only bit I really disagree with is the part about starts made/missed - if only because we're talking about one play. Some of that could be determined by game planning, matchups, personnel, etc. I think you're right to point out there's a loose correlation, but I don't think it means the coaches sense decline is on the verge. It could be they were knicked up, which you already addressed in a more meaningful way when showing the percentage of games played in the section before. It may be obvious that production will decline in the coming years, but that's not my takeaway based only on the first offensive play of any game.