Co-signing for a card is a good way - but make sure you have access to actually make sure they are paying it etc.

I had my own (in my own name, not an authorized user) with my credit union when I was 16 and by the time I was married I had a credit score over 800.

My credit score was actually higher than my wife's at the time, and she had worked full time with a normal salary, paid off a car loan, etc.