As in 6,000 years old, or whatever, it would completely change the theory of Evolution.
But if evolution is not, in fact, a fact, it doesn't matter how old the earth is. They're two different issues, just like abiogenesis--the origin of life--and evolution are two different issues.
You'll respond that there's no way the Earth is that young, but my point still holds. Evolution only works if the Earth is old enough to allow its gradual effects. Otherwise, it's just a good crack at figuring out where all those bones and similar phenotypes come from.
Good thing that it's been proven, over and over, that the earth IS that old, huh?
I am not yet born, console me.
I fear that the human race may with tall walls wall me,
with strong drugs dope me, with wise lies lure me,
on black racks rack me, in blood-baths roll me.