well, they don't neccessarily prove an old earth, just that geologic column was laid down sequentially -- ie: the law of superposition.
but it is distinctly hard to swallow the yec line with knowledge that each stratum was laid down independently, sequentially. and when you know that strata do not exactly form quickly... you also end up with the conclusion that geologic column represents a timeline of a portion of the history of the earth, something yec's don't like.
i've never seen a yec come up with a good explanation for angular unconformities, especially the ones that involve distortions of solid rock, but then again, i've never seen a yec come with a good explanation for anything. i have seen the "flood" thing tried, but that seems to be the magic yec answer for everything.
it just looks like those principles don't apply to unconformities.
the reason the law of superposition is a law instead of a hypothesis is because of angular unconformities. they are what prove it beyond any shadow of doubt.
in order for an angular unconformity to exist, that layer and all below it need to be solidified before they are turned on their sides. the layers on top then must be formed after this occurence, with time for weathering in between.
because angular unconformities are local and happen at many different levels, it tells us that the geologic column must have been laid down sequentially. we can talk about time later, but this is not the sort of event we see in a flood plain. floods always make very disturbed-looking, semi-homogenous single layers. they do not separate, sediment, turn over, sediment some more, and repeat. they make one layer.