Fold mountains are created when two tectonic plates collide and the edges of these plates "fold" due to the tremendous push force between them. They are very common and are usually made up of sedimentary rocks. These sedimentary rocks take on the appearance of wrinkled, crumpled fabrics being pushed together. They rise up and form fold mountains. Scientists classify fold mountains into either young-fold mountains or old-fold mountains according to their age.
Young-fold mountains are between 10 and 25 million years old, e.g. Rocky Mountains, Himalayans.
Old-fold mountains are greater than 200 million years old, e.g. Ural Mountains, Appalachian Mountains.
Interestingly, the Appalachians are older but much smaller than the Himalayans. Most people don't realize that many, many years ago the Appalachian Mountains towered over the current Himalayans.