PRE-TEENS & TEENS
Calcium is a necessity for building strong bones!  It is better absorbed through food than supplements.  Some great sources include low fat milk, low fat yogurt and cheese.
Iron is crucial for carrying oxygen to your blood cells.  In fact, a low iron count can cause you to feel tired, and has even been proven to affect students' math scores.  Iron is, again, better absorbed through food than supplements at this stage in life.  Red meat is best, but iron-fortified cereals, beans and spinach are also great sources.  All of these are even more effective when combined with Vitamin C or citrus fruits and/or juices.

MOM & DAD
Keeping healthy levels of folic acid can prevent birth defects in children and even lower your own risk of heart disease.  Folic acid is also best absorbed through food, though supplements are good for women who are planning to become pregnant.
VItamin D has been found to reduce the risk of several forms of cancer by 30-50%, lower your risk of death from any cause, and can help your body absorb calcium.  Still, 74% of Americans don't have enough in their diets.  Supplements are better in this case, if for no other reason than there are very few foods that naturally contain Vitamin D.  Either take 1000 IU/day or D3 of Vitamin D, or spend about 15 minutes in unobstructed sunshine -- our skin can naturally absorb Vitamin D from sunlight.

GRANDMA & GRANDPA
Calcium and Vitamin D are two of the most crucial vitamins at this stage in life because they help to prevent or ease osteoporosis.  Supplements are a fine source of both calcium and Vitamin D for older adults.  Most nutritionists recommend 1200 mg/day of Calcium Nitrate *AND* 1000 mg/day of Vitamin D.