Can heart blockage or coronary artery disease occur at age 25?

Yes but uncommon. Although uncommon, it is usually due to a congenital cause (conditions that one is born with). Classic atherosclerotic plaque is unlikely unless the patient has a severe form of family hyperlipidemia, type 1 diabetes, heavy smoker, Cocaine user, etc. Abnormal position of the coronary arteries is a rare but potential lethal cause of a heart attack.