Last I heard it was 4.55 billion years old +/- 2%
The point of the layers is that the ages are not based on any scientific theories, just counts. That they corroborate the measurements made by means that are based on theory (ie - all radiometric methods) means that the theories behind those measurements are validated for general use in dating other objects -- like the full age of the earth.
Any C'ist that would argue that they cannot be extended beyond the validating data would have to show what mechanism would change the theoretical results, ie - why would the rate of radioactive decay suddenly change?
Of course YEC's are SOL with the data from the lake algae, let alone the calcite layers. Someone mentioned Evo's being afraid of YECers a while back ... humorous.
we are limited in our ability to understand
by our ability to understand