From: Coronavirus: How Sick Will You Get?, Mercury News, 1 April 2020

Age may be more a marker of underlying disease than age itself:

This reference to “cytokine storm” was notable. It’s thought to be why so many young, healthy people died during the Spanish flu.*

Doctors are discovering that nine or 10 days into the illness, there’s a fork in the road. In most people, the immune system launches a carefully calibrated and effective response, so they recover. But in others, the immune response is too aggressive, triggering massive inflammation in what’s called a “cytokine storm.” Immune cells are overproduced and flood into the lungs, making it hard to breathe and leading to often fatal Acute Respiratory Distress Syndrome. Those people develop sepsis, then acute kidney and heart damage. By day 20, they may be dead.

Why does the immune system misbehave? One reason may be age. As we get older, our immune response grows less accurate. It doesn’t respond as effectively, and it is not as well regulated. Genetics may also play a role.

* Spanish flu, from Wikipedia:

The Spanish flu was an unusually deadly influenza pandemic. Lasting from January 1918 to December 1920, it infected 500 million people – about a quarter of the world’s population at the time. The death toll is estimated to have been anywhere from 17 million to 50 million, and possibly as high as 100 million, making it one of the deadliest pandemics in human history.

Most influenza outbreaks disproportionately kill the very young and the very old, with a higher survival rate for those in between, but the Spanish flu pandemic resulted in a higher than expected mortality rate for young adults. Some analyses have shown the virus to be particularly deadly because it triggers a cytokine storm, which ravages the stronger immune system of young adults.

Both a strong immune response and a weak response can be deadly.