The consensus among scientists is that the Appalachian Mountains are the oldest mountains in the world. However, there is not absolute certainty.
Scientists point to the crystalline rocks found in the Appalachian Mountains as the primary reason for giving it the title of the oldest. These rocks are between 1.1 billion and 540 million years old. The pressure and heat necessary to create these rocks would have wiped out any primitive life in the area at that time, so these crystalline rocks do not contain any fossils. The time period in which these rocks were formed is known as the Precambrian Era.