Nobody actually knows what causes kidney stones, in terms of diet. There is anecdotal evidence that suggests some possibilities, but nothing conclusive (unless that has changed in the last year or so, since I last researched it.) The only thing that we know for sure about them is that people who get them usually end up with multiple throughout their life. Also, family history is a risk factor, as is being white, and over 30. But, as others have said it is not uncommon for people in their 20s to get them as well.