Young fold mountains, such as Mt Everest, have been formed in the last 65 million years.
Older ranges of fold mountains, such as the Cambrian and Cumbrian mountains, and the Scottish highlands, are less high due to erosion.
Examples of young fold mountains are the Himalayas, the Rockies, the Andes and the Alps.
They are mainly found in two linear belts: one down the west coast of America and the other from the Mediterranean through south Asia, to Indonesia.
Fold Mountain Formation
Collision Boundaries - e.g. the Himalayas in Asia
Where an area of sea separates two continental plates, sediments settle on the sea floor in depression called geosynclines. These sediments gradually become compressed into sedimentary rock.
When two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.
Eventually the sedimentary rock appears above sea level as a range of fold mountains.
Where the rocks are folded upwards, forming mountain chains, they are called anticlines. Where the rocks are folded downwards, they bulge into the mantle as mountain roots and are called synclines.
(Remember that anticline/syncline is the opposite from what one'd expect. It's called an anticline, because the layers of rock slope downwards from the crest. In a syncline, the rock layers slope upwards.)
Destructive Plate Boundaries - e.g. the Andes in South America
Where sections of oceanic crust and continental crust are driven together, denser oceanic crust is subducted beneath continental crust into the mantle.
As the oceanic crust is subducted, sedimentary rock layers deposited on the ocean floor are scraped off, piling up onto the continent.
Sediment is compressed and folded in the process.
Eventually, the entire ocean floor is subducted and the two sections of continental crust collide.
Continental crust is to buoyant to subduct, so plate material is no longer destroyed.
These vary slightly because the fold mountains were not all formed in the same way.