Even though death rate is calculated based on the total population, it makes more sense to segment it between age groups and pre-existing conditions. We know that the chance of dying after contracting coronavirus increases with age, with the most vulnerable people above 80. We also know that majority of the death happened among people with pre-existing conditions (such as cancer, organ malfunctioning, diabetes, etc.).
This happens because of the immunity weakness. With age, immune system becomes weaker. As well as with ongoing medical condition. In other words, for people with weaker immune additional blow that SARS-CoV-2 brings is too much to take.