A good way to gauge sodium intake for kids and adults alike is to compare sodium to calories. Up to 2400mg in a 200calorie diet for an adult means roughly 1.2mg of Na per calorie. That would work for a 12-year-old, but it would depend on the 12-year-old: what sex, what size, how active? The absolute amount of recommended sodium would vary somewhat with the absolute amount of food, best measured as total calories.
That said, the Dietary Reference Intakes from the Institute of Medicine provide a recommended intake level based on age and sex alone. For both boys and girls this age, they recommend an upper limit of 2.3g of sodium per day:
http://www.nal.usda.gov/fnic/DRI//DRI_Water/269-423.pdf