From the given information, it can be concluded that the fast-food retailer is most likely to follow demographic market segmentation.
Demographic segmentation involves dividing a market based on demographic variables such as age, gender, income, education level, occupation, family size, and other similar factors. In this case, the fast-food retailer has observed that his sales orders do not show much variety with respect to the store location, but instead, the needs of his customers are age-related. This suggests that the retailer is likely using age as a demographic variable to segment his market and tailor his products and marketing efforts to different age groups.