文段：Note that the reference group for the age categories is Age: 18–24. The reference group for the country dummies is the United States for the OECD countries and China for the non-OECD countries. As Table 1 shows, the United States is a young country that in large part does not face the problem of an aging population like many of the other OECD countries. China is the largest non-OECD country, and unlike the other nonOECD countries, China will experience an aging problem in the future, especially in terms of actual numbers