Is the heart disease (cad) risk for a 20 y/o significantly increased by lack of exercise and unhealthy diet when there are no other risk factors?

Yes & no: At 20yo, overall risk for atherosclerotic heart disease is slim to none in most individuals, even those w/family history of early heart attack. Therefore, while bad lifestyle increases risk, it's not a tremendous increase in absolute risk. However, heart disease from cholesterol is different from heart disease & sudden death due to abnormal rhythm. That's why family docs ask about ur family histor.