What is the appropriate age to start checking for diet problems

Is there a specific age that one should be to start looking at the diet as the source of some of the problems our son is facing. He is currently 10 years of age and does not really 'seem' allergic to any foods, although I am not sure really which foods are/can be bad for him. I have been reading a lot especially some of the information that Jenny McCarthy talks about with her son and the diet changes that she made that caused a huge change. I am would like to know what is the best place/test to start with and what the approximate costs will be.