The major difference between young and old mountains is the level of erosion within an entire mountain range. For example, the Rocky Mountains and Himalayan Mountains are young because they are still rugged, with very few signs of erosion. These mountains are still uplifted due to tectonic forces. The Appalachian Mountains and Ozark Mountains have flatter tops and have been eroded longer, so they are older.
As soon as they appear above ground, mountains begin to fracture and dissolve due to rain, snow and other weather phenomena The estimated age of the Appalachian Mountains is hundreds of millions of years. The Himalayan Mountains, the tallest mountains in the world, are believed to be about 30 million years old, and they continue to be thrust upward by tectonic action.
Usually, taller mountains are younger than shorter ones because the shorter ones have endured more erosion over time. The appearance of mountains gives clues as to the age. The Rocky Mountains are jagged, snow-capped and steep. The Appalachian Mountains are shorter, rounded and topped with trees. The highest peak in the Rocky Mountains is Mount Elbert in Colorado at 14,440 feet. The highest peak in the Appalachian Mountains is Mount Mitchell at 6,684 feet. Hence, the Appalachians are considered to be much older. Researchers estimate that erosion of tall peaks takes millions of years.