Surely, you remember a 3rd string QB (Yates) that had to start last season? Same scenario. Youth or vet you keep the player you think is best for team. Also, youth is not as much a factor for a RB.
You are making arguments against the wrong guy as I have been saying Grimes should make the team since before training camp.
My point with the youth thing is that Kubiak may not want that much youth collectively contributing. Doesn't matter that a RB can contribute faster at a young age than some other positions. It's still a collective thing.
You have AJ, and Schaub who aren't exactly the most healthy guys...
You could easily have a scenario where you have Martin, Jean, Yates and Grimes getting regular or significant playing time.
Instead of going with that much youth, Kubiak may want a vet like Forsett that has been around the game for a few years.