In Queens where I grew up, there are whole sections of cemeteries with grave after grave of young people who died suddenly in 1917/1918.
This was an odd one. Young and healthy people seemed to suffer worse. IIRC, the virus utself attacks the white blood cells.
So people who are healthy and have strong immune systems produce the most amount of white blood cells. And give the virus lots of lunch.
The other thing was the fantastic quickness it killed folks. It was not unusual for a person to wake up in the morning feeling fine and be cold and dead before midnight that same day.