The phylogenetic results, combined with these other lines of evidence, suggest that the high mortality in 1918 among adults aged ∼20 to ∼40 y may have been due primarily to their childhood exposure to a doubly heterosubtypic putative H3N8 virus, which we estimate circulated from ∼1889–1900. All other age groups (except immunologically naive infants) were likely partially protected by childhood exposure to N1 and/or H1-related antigens.

The Spanish flu apparently had the N1 complex present, to which the 20-40y population wasn’t exposed. At least that’s my limited understanding after skimming the paper.

They argue that people born before 1889 (?) were exposed to a virus similar to the Spanish flu, whereas people born in a timeframe directly thereafter were not. They experienced a different virus that wasn’t as closely related. Thus their antibodies weren’t as prepared.