Well I've had a look around and not unexpectedly I can't come up with any studies on heart attacks by age group in Horses. However I did get some info that may be of interest.
Horses don't have heart attacks like people do. They don't get aerthrosclerosis ( Clogging, narrowing, and hardening of the body's large arteries ), which makes sense seeing as they're vegeterians,they tend to have ruptures of large vessels, aneurysms and (probably more often) arrhythmias(Abnormal heart rhythms)
Abnormal heart rhythms can lead to collapse or death but usually causes exercise intolerance. If ruptured aorta is the cause of death mucous membranes become pale from internal bleeding. Age also can weaken the contractions of the heart muscles. Some horses obviously have good strong hearts just like some people. Congestive heart failure which causes enlarged hearts is usually fatal. Signs include enlargement and rapid pulsing of the jugular veins,coughing,rapid breathing,swelling of legs,exercise intolerance and increased heart rate
It would appear to me that where a horse appears to be healthy and than dies suddenly the horse had an aneurysm that popped.