Favre was 40 when he was an overtime loss from going to a Super Bowl.
Both Rich Gannon and Kurt Warner went to Super Bowls at 37 (Gannon was the league MVP that year).
YA Tittle was the AP MVP and was the losing QB in the NFL Championships at age 37.
At Age 37, Steve Young passed for 4,170 yards, 36 TDs, and 12 INTs while rushing for 434 more yards.
Warren Moon at age 39 passed for 4,228 yards, 33 TDs, and 14 INTs.
Personally, I find it hard to judge when Brady's age will become a factor. The only guy I think is a good test bed is Favre. With today's conditioning and medicine, guys like Montana and Young might have gone into their forties before slowing down. There was a draught of elite QBs between Favre and Brady and Manning so unfortunately there isn't a lot of test subjects to determine how long elite QBs of this era could last.
Many elite and very good QBs in the past had career ending injuries (or degenerative injuries) that would only hold a QB of today out for a few weeks or months. We are seeing players at every position playing far longer careers than it was ever dreamed of a few decades ago. If Ray Lewis played in the 70s or 80s, his career probably would have ended 5-7 years sooner than it did. Brady's career would have ended in 2008 if the medical science was still the way it was back in the 1980s.
I think among the current crop of elite/top 10 QBs, we will see several of them play at a high level into their early to mid 40s. Will it be Brady? Who knows? But I guarantee you that you will see it with at least some of the top QBs of today.