When I was four years old, my parents opened a major credit card and made me an authorized co-signer. Although I wasn't allowed to use the credit card until I was 16 (and then, only for emergencies), and wasn't made the primary account holder until I was 18, it built up my credit like crazy. Having a credit card open and completely paid off for almost 15 years before I ever used it increased my credit score. Now, when I go to get pre-approved for large purchases, I have a much higher credit score than other people my age.
Parents, if you have good credit habits, open a credit card for your child and keep it paid off until they are old enough -- or responsible enough, whichever comes first -- to use it. Their credit score will thank you!