Boomer Esiason was a 7th string QB (you read that correctly) at Maryland (per interview on WEEI). Tom Brady was Michigan's starting quarterback in 1998 and 1999, but struggled to hold off then-wunderkind, Drew Henson (the first of the entitled who I can recall). Brady was also once 7th on the depth chart. Things have a way of working out.
The problem with college nowadays is the sense of entitlement. If a freshman is not on the two-deep or is redshirted, they immediately start targeting schools to transfer. You may remember that UConn had such a player on it's roster earlier this year.
Granted, I started following college football in earnest in the early 90's, but up until Peyton Manning (who was forced into the game as a Freshman), I don't remember reading about or hearing of college QB's who have started all 4 years in college. It never really happened. It couldn't happen prior to 1972, but why has there only been a single Freshman Heisman Trophy winner in 41 years? Because, by and large, the powers that be think it should happen either. 18-19 year olds don't really have the maturity on the whole to handle the accolades associate with such early success and Johnny Manziel is anecdotal evidence to the affirmative.