What is the probablity of a 16 year old to have a heart attack/stroke?

Very unlikely: Unless, the 16 yo has a family history of homocysteinuria, or has an injury causing an arterial dissection, or has a genetic disorder such as mthfr tt mutations. Congenital heart disorders also an issue. But, assuming the adolescent does not use Cocaine or amphetamines, risk is very low indeed.

Rarely : There are a few conditions where a 16 yo could have a heart attack or stroke similar to a 60 yo.(progeria, kawasaki) there are also various congenital heart defects where a teen could develop a sudden rhythm disturbance and present a similar set of symptoms.Sickle cell kids can have strokes. Added together these are still rare, but worthy of consideration when a teen goes down suddenly.

Very low: Congenital anomalies aortic root, aortic valvular disease aberrant coronary anomalous coronary origin but as another has noted: Cocaine and other drugs of abuse are more likely hyperlipidemia.