Why older mountains less height than younger mountains

Oder mountains Aravallis and Andes are less height than younger mountains Himalayas due to denudation process.The Denudation process means weathering and erosion.The breaking of rocks into small rocks is called weathering,the removal of top surface is called erosion.Due to this activity the older mountains losses the top layers for many years.So,the older mountains is less height than younger mountains.