Based on avg career length of a QB.
example: Romo got 9 seasons. Staubach 10. Aikman 10 1/2
Low number on Dak is 7. High is 12. Take the avg.
Romo started at age 26, Staubach at 29. So, take that into your equations, as older age does play a role. Dak started at a much earlier age and has avoided serious injuries to his back, collarbone and brain so far.
Aikman retired due to his concussion issues and well obviously he got cut. If Dallas wasn't in "Cap hell". maybe his career could of been longer if he didn't sustain any more concussions.
In the modern age of the NFL, most QB's don't have a short peak, if they are good QBs and avoid injury.
Alex Smith played for 13+ years and would of for a few more years if not for a nasty injury.