Food labels have greatly enhanced our ability to choose healthier diets. Americans (and others around the world) have problems with several food-related disorders, for examples overweight/obesity, high blood pressure (from too much salt or overweight), diabetes, osteoporosis (thin bones), constipation, colon cancer (too little fiber), etc. Using food labels allows us to choose foods with lower fat, lower saturated fat, lower salt, more calcium, more fiber, or simply fewer calories. We often use the labels to choose between two or more similar options (such as similar cereals, breads, juices or other drinks).
Teaching children about food labels during childhood can be an effective way to help children learn about the foods they eat and help them learn to make healthy choices. Children learn early what to eat from example of those around them, mostly first from parents or other daily providers. From this starting point, teaching children to read labels and what the meaning(s) of the nutrients listed is a wonderful way to help children (and even adolescents) have some choices on their dietary intake. Obviously we don't want to make the kids hyper about this; we do not wish to create eating disorders by placing too much emphasis on everything that is eaten, but making healthy choices can be fun and educational in addition to promoting healthy lifestyles.
School foods can influence what they eat, and many schools are now also showing the nutrient content of the foods being served. Similarly restaurants are showing calories and other nutrients on menus (kids are often surprised at the 1000's of calories found in servings of some meals!).
The age to start having kids read lables depends on the child's ability to read and interest in food -- and may be of particular interest if they are involved in food preparation or if they are shopping with parents for food. Others may start later. Interestingly some recent studies show that 5-6th grade children who pay more attention to food labels have lower BMI (body mass index, a measure of weight and height that relates to overweight) compared with those who do not pay as much attention to labels
I have included a couple of related web sites below, one from the NIH and the other describing food labeling from the FDA.