Taken together, these considerations imply hospitalization rate for < 40yo should be far below the 10-20% figure. (On the flip side, many of the currently infected who have not yet been hospitalized will be hospitalized in the future.)

I don't know the hospitalization rate for < 40yo, but the death rate is thought to be around 0.1%. It would be very surprising if the hospitalization rate were more than a couple percent.