If people mean anything at all by the expression "untimely death", they must believe that some deaths run on a better schedule than others. Death in old age is rarely called untimely- a long life is thought to be a full one. But with the passing of a young person, one assumes that the best years lay ahead and the measure of that life was still to be taken.
History denies this, of course. Among prominent summer deaths, one recalls those of Marilyn Monroe and James Deans, whose lives seemed equally brief and complete. Writers cannot bear the fact that poet John Keats died at 26, and only half playfully judge their own lives as failures when they pass that year. The idea that the life cut short is unfulfilled is illogical because lives are measured by the impressions they leave on the world and by their intensity and virtue.