Question

Why are himalayas called young fold mountains

Answer 1

Himalayas called young fold mountain because The Himalayas were formed in the recent past(according to geological timescale; some 2 million years ago) by the coliding of the Indian landmass with central Asia.  Note that India was earlier an is land that had budded off form the continent of Australia. This is why they are called younger than other mountains.
Fold mountains are formed by the collision between two landmasses(technically, the tectonic plates that make up the Earth's crust), so that the point of collision rises high and forms a mountain.

Answer 2

Himalayas are found to be formed 'very recently' when the Indian Plate collided with the Tibetan Plate (See Plate Tectonics) during the period called Paleocene-Eocene (50 million years ago). This collision of two continental plates formed Orogens ormountains. ... Hence Young Fold Mountains