I spent some time with Chris Morris last week. I've known Chris for a long time and he has, over the years, developed an unparalleled knowledge of the science of whiskey-making. That includes, of course, maturation.
I absorbed only a very small part of what he told me (but I rolled tape). One thing that stuck, however, is that when aging a spirit in new, charred oak barrels, the color comes first, the flavor comes later, so "ooooh, it's getting dark," should be tempered with "the flavor takes more time."
We sampled a barrel that was only 6 months in wood. It didn't look much different than a 4-year-old, but it tasted like white dog.