Correct, late teens to mid thirties age bracket... pushed a healthy immune system into overdrive, fluid filled lungs, heart failure in hours.. pretty rough
When the virus first hit in 1918 it was the usual victims that died-the young and elderly. In the second wave the healthy strong demographic you described began dropping like flies. Recent thought I’ve read indicates it may not have been a mutating virus after all. Debate is ongoing on many aspects of the Spanish Flu pandemic.