The best time to begin preventive anti-aging techniques is the 20’s and the best way to do this is to defend against your skin’s biggest stressor, namely sunlight. As a general recommendation, it is always a wise choice to use broad-spectrum sunscreen if you know you are going to be in the sun for extended periods of time.
As far as “anti-aging” creams go, it is best to take the manufacturers claims with a grain of salt. Since these products are cosmetics, the FDA’s regulations are rather lax in comparison with other drugs. The best way to minimize the oxidation effects caused by the sun and other pollutants is to regularly apply a moisturizer/antioxidant mixture that will help you reenergize your damaged skin cells.