My father who is 62 years old is diagnosed with mild cerebral cortical atrophy after an mri. what is its treatment and is it fatal.

None: Mild cerebral atrophy usually is a condition that arises from aging. There is no known treatment for this, it is not fatal.

Can be normal: As we grow older, we lose brain mass, some at rates faster than others i.e. alcoholics, people with vascular disease, Alzheimer disease. This is a normal part of the aging process and is likely a normal finding based on your father's stated age. Taking care of risk factors such as high cholesterol, hypertension, and diabete would be most important, assuming your father is physically fine currently