Actually the second go around that was really deadly killed younger, healthier people by kicking the immune system into overdrive and overwhelming the body from within. It’s called a Cytokine storm. Older people don’t have such a good immune system so it wasn’t as virulent.
http://en.m.wikipedia.org/wiki/Cytokine_storm
My grandpa came home from World War I and his three younger sisters had all died. They were teens IIRC.
Exactly right...at least according to this book as well
http://smile.amazon.com/Great-Influenza-Deadliest-Pandemic-History/dp/0143036491/ref=sr_1_1?s=books&ie=UTF8&qid=1413227369&sr=1-1&keywords=the+great+influenza