I think the principal point to be made about the age of the earth is not that it somehow "proves" evolution, but that it was originally raised as an objection to evolution. Darwin's detractors said, in effect: "Very nice theory, Mr. Darwin, but the earth isn't old enough for all that evolving, so that shows you're wrong." Well, so much for that objection. (Except for the YEC gang, but they're totally nuts.)
It's a matter of semantics. Darwin's theory required the earth to be several hundred years old. That could be rephrased to say evolution predicted an earth at least that old -- the first accurate prediction based on an observed rate of change. Other predictions -- based on the saltiness of the oceans or the gravitational collapse of the sun -- were off by great margins.