Healthy young adults below age 50 survive Covid-19 better than influenza virus,
but 1 in 3 young adults have a pre-existing condition that could make Covid-19 fatal or permanently injurious.

https://medicalxpress.com/news/2020-07-young-adults-higher-severe-covid.html

It seems to me that a young adult could have no pre-existing conditions mentioned in the article,
but if their immune system is circulating an above average amount of the allergy related Cytokine named IL-13
mentioned in a previous post
they could die from a “Cytokine Storm” over-reaction.