Final answer:
The principle that determines the age of rocks with the oldest layers at the bottom and the youngest at the top is known as the Law of Superposition.
Step-by-step explanation:
The principle used to determine the age of rocks by ordering them from oldest at the bottom to the youngest at the top is the Law of Superposition. This principle was first proposed by Nicolas Steno in 1669 and is a fundamental concept in relative dating techniques used by geologists. According to the Law of Superposition, in a sequence of undisturbed sedimentary rocks, the oldest layers are at the bottom, and the layers become progressively younger toward the top. This principle helps geologists and paleontologists to understand the chronological order of geological and fossil events as they appear in the strata.