Final answer:
The oldest desert with a rain shadow effect from the Andes and Coast Range is the Atacama Desert in Chile. The Andes Mountains cause the rain clouds to release their precipitation on the eastern slopes, leaving the western side dry.
Step-by-step explanation:
The oldest desert on Earth that has a rain shadow from the Andes and Coast Range is the Atacama Desert in Chile. The rain shadow effect is responsible for the extreme dryness of the Atacama.
The Andes Mountains, which are high at this latitude, cause the wind-blown rain clouds from the east to ascend in elevation and release their precipitation on the eastern slopes, preventing the western side of the Andes from receiving much or any rainfall.