In general, I like the idea of guys staying for a year at the lower levels.
Its a big shock to be on your own for the first time, paying your own bills, playing ball every day, and dealing with long bus rides. Its nice to give the guys a chance to get some stability. Get comfortable with the entire lifestyle change. That goes for the first couple years. Confidence is a fragile thing, and you don't want to promote a kid too soon when he's doing well, then challenge him and watch him crawl into a hole he can't get out of.
Once you get to AA though, IMO, all bets are off. Once you hit that level, if the talent is there, and you're having success, you're good to go. The maturity should be there, a certain level of experience has been accumulated, and obviously the talent is there(if you're at the level and doing well).
Of course, there are exceptions. Bryce Harper being one. I think he should at least spend the month of May in Hagerstown, but if he's lighting it up then, get him to Potomac.