Why the young were so vulnerable is seemingly still a mystery.
One theory you already suggest - debilitation by gassing (or trench life in general).
Another I've seen is that the particular 1918 strain induced immune-system "storms" that themselves caused body damage - a
healthy young immune system could actually be a liability. cf:
https://en.wikipedia.org/wiki/Cytokine_release_syndrome
These weren't really recognized and studied until transplants (and immune reactions to them) became common more than half a century later.