Sure, you might be able to point to a QB here or there who came on strong to become a consistently above-average player several years his starting career, but for every guy like that (Steve Young might be the most notable) I'll give you multiple Joey Harringtons, Kyle Bollers, Rex Grossmans, and David Carrs.
Alex Smith would have been on that list too, at least until the last couple seasons. He might be the exception to the rule. After Smith's phenomenal game Monday night in which he went 18 for 19 with three TDs, I wondered what really made the difference. A lot of credit has been given to second-year head coach Jim Harbaugh and a new offensive scheme. It's hard to argue otherwise, but I will anyway. Looking at his career trends, he began a significant and consistent improving trend immediately following his missed season of 2008. His improvement from 2009 to 2010 in terms of WPA, EPA, SR, and AYPA under Singletary was as big as his improvement in 2011 after Harbaugh took over. It's as if Smith has had two different careers, a brief, erratic, injury-plagued three-year span from 2005 to 2007, and a middling but improving span since 2009.
I won't pretend to follow the 49ers closely enough to understand the real reasons for his improvement any more than the average poster at Bleacher Report. He's been much healthier in recent seasons. The offensive line has solidified, and the receiver corps is now certainly much better than those led by an aging Isaac Bruce or Arnaz Battle. But I do think the narrative that Smith has been reborn under Harbaugh into an above-average QB is, if not mistaken, fairly shallow.
Smith's recent performance should also be put into context. Even now in the midst of what appears to be a career year, he ranks 14th in EPA per play and 21st in WPA per game. But he does rank near the top of the league in SR at 7th, thanks largely to Monday's game. (I should also note he is tied atop the Air Yards ranking.) Despite the attention heaped upon him, he's only made the jump from 'clear liability' to 'league average.' That's no small feat in such a competitive league and in an era of plentiful skilled QBs, but the real drivers of SF's success are their #1 running game and top ranked defense.
Here are Smith's career numbers. His full stats can be found here.
Who are other late bloomers? CFL guys don't count!