According to a response from Jack Daniel's marketing department to a direct question from my storeowner about what the difference is between Green and Black label:
The Green label is what is rejected by the master distiller when selecting barrel to blend for Black label. So, if it's younger, it not deliberate -- though, of course, since numerous barrels are blended together in a single 'batch' it may well be that younger ones are rejected more often than older ones. I don't know about that, and our respondant didn't address age (of course, JD had no age statement on it).
Oh, and by the way -- I subsequently confirmed this info independently when I ran across somebody who works at the distillery. He told me the same thing without prompting (Other than the question, "What is different about the Green labeled bottling?").