How many servings should a twelve year old eat?

When it comes to eating healthy, it's always a balance between WHAT and how MUCH you're eating.

The number of servings doesn't really say a thing. You could have heaps of fluffy salads which would take buckets to reach the required nutritional content. Or one could have a balanced meal where one plate for each meal will be all it takes.

A very crude guide line is to have half the plate with vegetables, 1/4 with protein(meat) and 1/4 with carbohydrates(pasta/potatoes/rice).

One such plate each meal shouldn't be too far off.