For much of 1918, a lot of people in their 20s getting the flu were also experiencing trench warfare, which can't be an optimal situation, both in terms of immediate self-care and co-occurring health conditions.
Older people were less vulnerable in the U.S. as well. The problem was that young healthy people were more likely to have an immune system overreaction. This applied to both genders.
(Source: John Barry's The Great Influenza)