In the United States, which mountain range is the youngest?

The Grand Teton mountain range has telltale signs showing they are not as old as the any other mountain range in the U.S. The peaks of the mountains are noticeably sharper and jagged, where the other ranges have smoother peaks.

The Teton mountains stretch across the states of Montana and Wyoming.

This mountain range is considered "adolescent" in age, and are believed to be 10 million years old.

The Appalachian mountain range is the "granddaddy in Noth America, at approximately more than 300 million years old.