Finally a topic worth starting a thread on. Thanks TFP.
Complicated for sure because there's so many variables. I think the most important thing is can the person retain information and translate it to the field. A good foundation from college on understanding defenses and the cause/effect routes have on them - is important but not the end all. If the player can learn they can still "get it" later. The speed of the NFL game is what's next to overcome. Learn, study, grow but if the speed of the NFL stops the player from processing mental into physical, that's a problem.
Today's NFL is not helping young QB's grow. Offensive coordinators leave for other jobs or get fired for lack of success, I'm guessing in 3 years or less on average. WR's come and go, RB's come and go, Lineman come and go, TE's come and go - so free agency movement is a huge issue with continuity. All the more reason the young QB has to be so prepared by himself. There's just not the 10 year WR and QB relationship anymore.
IMO (and I hate to say this because I really like McDermott), but the best chance a young QB has, is if the head coach is a great offensive mind. It helps reduce the impact of the OC leaving and their strong relationship keeps the team moving forward.