Hello,
As for heart attack we have seen the trend that the age is really decreasing when one gets chest pain and diagnosed as heart attack. But for chest pain as such there are numerous reasons. It is not right to exclude heart attack by oneself seeing the less age criteria. But for lower age group the more common causes should be ruled out - trauma, viral infection which may not give fever, costochondritis, muscle sprain. For such a long time chest pain its most likely to be costochondritis which is observed that 10-30% of children and adolescent has it. It gets relieved by itself in few days to months.
Hope this should ease you and hope for quick resolve.

Hello,
For your first question regarding the possibility of a 15 year old having a heart attack,the answer is yes.But,the pathology is usually different.It is usually secondary to some abnormalities in the clotting mechanism.There is a increased tendency to clot formation.Such a clot can cause a block within the coronary arteries(blood vessels of heart) and cause a heart attack. Excess homocysteine in blood is one such condition.
For your second question, though the definite answer could not be given,but the chances of it being a heart attack is almost nil.Other causes like aneamia need to be ruled out.Echocardiogram of the heart may be necessary to rule out other pathologies in the heart,especially the valvular diseases.Valvular disease of the heart do present with shortness of breath,palpitations,chest tightness etc.Certain septal defects of the heart can also present at this age.Musculoskeletal pain is another likely cause.
Hope to have clarified.