This isn't difficult stuff, IMHO. A small age difference is no big deal. A year, sure it's a factor, but can clearly be overcome by other factors. But what about two years? Three? Five? Clearly if there are two prospects that are pretty close tools-wise, a large age difference argues for the younger prospect.
I realize that high school draftees won't have a five-year age difference. But two years is possible; my son has two classmates, both high-level athletes, born in August -- one in 1999 and one in 2001. So one will be 17 when he graduates (and turn 18 in August) and the other will be 19 when he graduates (and turn 20 in August). To me, if they are close tools-wise and performance-wise, the 17 year old is more impressive.