There's a bunch of different studies, mostly focused on the results from China, but there's also a few things trickling out from other seriously affected areas.

Basically 15% of people (of ANY age) who present symptoms get so sick as to require hospitalization. Younger people have strong bodies and with an ICU bed, drugs, and proper medical care they are able to recover over the course of a few weeks. But without that intervention, they have a good chance of dying.

That is what happened in the last days of the Wuhan crisis (peak fatality rate: 5%). It is what is happening now in Italy (fatality rate: 8% and climbing) and Iran. That could be the rest of the EU in a week, and the USA a week after.

When the healthcare system is over capacity, the "needs hospitalization" number becomes the fatality rate.