We did know by April at least (IIRC basically right after that cruise ship docked). Wikipedia for example [0] had CFR stats broken down by age group showing it was basically a nuisance to people under 50 and only breaking 1% mortality for people over 60 (i.e. it didn't affect children or the working population, so closing schools was known at the time to be a stupid idea. IIRC Fox News were pointing this out at the time), and IIRC at the time the CDC was estimating ~30% asymptomatic cases, so an even lower IFR.

[0] https://en.wikipedia.org/w/index.php?title=COVID-19&oldid=94...