No, you don't know EXACTLY how old it was. You know APPROXIMATELY how old it was.
And rock and earth aren't trees. The accuracy becomes less and less accurate the older things are.
Now you're just playing with words. Did you expect the age down to the nanosecond? Sure, when things get older the spread of readings may get larger, but in that case you're usually concerned with how many millions of years, not how many years. The percentage error doesn't vary that much, so within the constraints of a particular test they ARE very accurate.