What is the oldest mountain range in North America?

Answer 1

Rocky Mountains.

Answer 2

From Wikipedia

The St. Francois Mountains were formed by volcanic and intrusive activity 1.485 billion (1.485 x 109) years ago.[3]By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 140 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.

There are several mountain ranges often cited as the oldest in North America. The Wichita Mountains in southwestern Oklahoma began forming 550 to 600 million years ago. Similarly, the Unwharrie Mountains in North Carolina began forming approximately 500 million years ago. The Appalachian Mountains began forming 470 million years ago. However, the formation of the Appalachians brought older rocks to the surface. Unlike the older Wichita Mountains, there are rocks over 1 billion years old on the surface with rocks as old as 1.3 billion years old found. Like the Appalachians, the Arbuckle Mountains in south-central Oklahoma were formed shortly after the Appalachians, but have some rocks as old as 1.4 billion years of age. All four of the ranges listed above are thought to once have had peaks reaching near or above 20,000 feet from sea level.