Premature death
\ˌpriməˈʧʊr\ \dɛθ\
Premature death is effectively death resulting from something other than old age. Causes of premature death include disease (e.g. cancer, heart disease), murder, suicide and malnutrition.
However, the definition of old age (and so premature death) is not precise. Despite old age being a recognized cause of death, its use in death certificates can be inconsistent. Some would say that old age begins at 65, whilst others would argue that old age begins when one is approaching the average life expectancy – i.e. around 75 to 80 (perhaps).
The latter meaning is a useful reference point for other quantities like rates of mortality improvements, which have tended to be higher at younger ages than in old age, or as a geriatric.
In the interests of using a common language, the UK Office of National Statistics’ definition of premature death, as a lifespan of less than 75 years, seems a sensible reference point.