Career length always makes me crazy (beware, rant to follow). I don't think it tells you anything useful.
A lot of guys come up, are busts and gone in a year, 2 or 3. That doesn't help arrive at a useful calculation on how long a starter plays. Marauder has the part right about better guys playing longer. To me, you come into the league around 23. RBs are good until roughly 30 and play until maybe 32 while winding down. That's 7-8 years of good production. WRs often get another 5 years of high level play. QBs can typically go maybe 3 after WRs, so they get a very rough 16? But different guys are just different guys. Some wear out sooner than others. Some get hurt and retire, others get hurt a lot and are beaten down. Some seem to never get hurt and glide along forever, others have concussions, get too stiff, some fade from losing too much speed, gradually or quickly ... IMO, its too wide a range to call for the individiual except by looking at him and his skills erosion. But absent injuries that cripple, I think you can expect to get 7 good years out of a starting RB, a sure 10-11 from a WR and a solid 14 from a starting QB, with the average stud running a year or two better than that.