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
etc.Certain septal defects of the heart can also present at this age.Musculoskeletal pain
is another likely cause.
Hope to have clarified.