I think in general, you just can't predict what is better for a young QB, starting early or holding back. We have examples for both that show one thing or the other. Manning started early and was successful. Pennington started and was as well. It may be that if they had done the opposite we would have had similar results, or maybe some QBs respond to one situation better than they would if they did the other. We just can't know.
For me, it seems clear that you just start the best player. Whether he be a rookie or a veteran, who ever gives you the best chance to win you start. This isn't a developmental league here, the job is to win games. Obviously the dynamic changes when you things aren't working and you have to make a guess and try a change or if the team is out of the playoffs. Until then, you put your best player out on the field, you can't know what is better for a young qb and winning should always be paramount.