A. You are right that infants should not be switched to cow's milk before they are a year old. The main reason behind this recommendation is that cow's milk doesn't have any iron in it, and so replacing breastmilk or an iron fortified infant formula with regular cow's milk before an infant is a year old would likely lead to your baby developing iron deficiency anemia. Some infants also have problems digesting large amounts of cow's milk.
If your infant is not at risk for food allergies, you can give some milk based products before they are a year old though. In fact, the American Academy of Pediatrics, in their Guide to Your Child's Nutrition, recommends introducing yogurt and cheese once a child is 9-12 months old.
If your baby is at risk for food allergies, like if they have eczema, asthma, allergies, food allergies or if they have other family members with these conditions, then you should avoid all dairy products until your baby is at least a year old.