The Himalayan mountains began forming in the late Cretaceous-Tertiary Period (70my) and continue to increase in elevation even today. That makes them among the youngest of mountain chains on the earth. They are fold mountains because of the compressive forces involved in the collision of the two continental plates involved, the Asian and the IndoAustralian.
Compare the Himalayan mountains with the Appalachian mountains (300my). The Appalachian are "old fold mountains" having been formed hundreds of millions of years ago. And, although they reached a height taller than Mt. Everest in the Himalayan mountains, the Appalachian mountains have eroded so that they are less than 10,000 feet tall today.