I think one of the worst things you can do is assign an age to any player position. If you are playing this game correctly then you are WATCHING what your players are doing and you have a sense. With that said, for a less-experienced owner wanting a starting point to begin watching closer, I like Bruce's guideline a lot better than AZ professors.
The golden rule is "better a year early than a year late". Self explanatory. If you are on the fence, then I would pull the trigger early and take my lumps that way because mitigating the loss is still better than zero (which is exactly what you will get at some point).
Overall, I think when you are dealing with the more proven players (the stars of the NFL), that you're actually better off being a late-buyer than a late-seller. Guys like AJ are getting written off prematurely IMO. He just happens to have a "3" in front of his age and he got hurt. People are jumping to a conclusion. There is little hope of getting anything even close to his value, but his true value is he is a guy who could easily be a top 1-5 WR this year. I'm more inclined to buy than sell, given that scenario and that is not dependent on whether I am a contender or not. If I am a contender, I buy. If I'm not, I still buy because I know that if I buy him low enough (basically, if I'm right on this one), that, worst case scenario, I will still trade him for more than I paid sometime in November when a contender comes knocking on the door of my 2-7 team. Again, this is only my theory on proven players because, let's face it, if Titus Young is having a top 15 season this year and AJ is having a top 12 season...and my team sucks...then a contender is going to be drawn to the name AJ, and also know that the price to buy Titus will be more because of age, regardless of the fact that AJ is having the better season.
QBs, I think, are immune to the aging out theory. If you have a QB that you are still interested in for your fantasy team when he is in his early 30's, then that usually means that he has been pretty good for a long time. Players like that don't tend to drop off the proverbial cliff unless there is a career-ending injury (which could happen to ANY player at any time). Will there be a time when your player DOES retire or whatever? Sure. But you are playing in a dynasty. Two years ago (going on 3), I bought a 32(?) year old Brady in dynasty. The guy laughed at me when I gave him a young QB (26-27 at the time) AND a 1st. Well, the QB didn't ascend as he thought and isn't Brady and the 1st was a very late pick. And I coudn't be happier. I don't tell the story to say "I did good"..I tell it to illustrate what this guy was thinking (because he told me). He thought the age of Brady was working against him. But like I said, I think you can trust in the proven guys as long as they play.
The idea of a dynasty isn't purely to pretend you are on Wall Street. The best Dynasty teams are "layered". You have your vets, and you have developmental players, and you have your new infusion. In my experience, the best long-running dynasty teams are always the ones with a strong mix and if you ask those guys, they will generally tell you that they plan to have a shot at winning in any given year (which IS kind of the point, right?) and they rarely get into these movements where they are either completely all-in right now or thinking "just wait until two years from now".