Aside from the other points people have made, the main hypothesis is that the 1918 flu was similar to previous pandemics which the older were exposed to when they were young, so they already had some resistance. The 20-40 year olds were unlucky in that this was their first exposure.
https://pubmed.ncbi.nlm.nih.gov/30349811-back-to-the-future-...
Aside from the other points people have made, the main hypothesis is that the 1918 flu was similar to previous pandemics which the older were exposed to when they were young, so they already had some resistance. The 20-40 year olds were unlucky in that this was their first exposure.