Dairy actually boosts your health as you age. It’s a fantastic source of calcium, which keeps your bones healthy and strong, and offers protein to nourish your muscles. Where dairy can hurt you after age 40, though, is in its fat content. The saturated fat in dairy increases harmful blood cholesterol levels, which can contribute to heart disease, and also contributes to chronic inflammation, which triggers cellular aging. Some fatty forms of dairy like ice cream or full-fat yogurt also contain sugar, which can make you gain weight.