How can one rock layer with an absolute date can narrow the ages of rocks above and below it.

The deeper the layer is the older it is and say it's the top layer it would be 1 year old but say you had a rock with a total of 15 layers and they ask you what the youngest and oldest layer is the youngest would be 1 year old and the oldest would be 15 but say they ask how old the 6th layer is it would be 6 years old