Translation:The old man had amnesia when he died.
Hukommelse - memory + s + tab - loss? Hukommelse seems to be related to the verb at huske - to remember.
Hukommelse comes from "at komme i hug" - to come into mind. I don't think that "hug" is used in modern Danish. Compare with the names of Odin's ravens Huginn and Muninn (norse: mind and memory).
You use da for single points in the past, and når for everything else, basically. I see you're learning German, so you will run into the same pattern with "als" versus "wenn".
@RyagonIV Bless you for answering this! Now I can stop asking for clarification of when to use når or da. Have a Lingot.
@Zet955992 I have been posting this same question every time I encounter a sentence with either når or da.
The old man had memory loss when he died. There's not much context, but in one way, it could mean he doesn't remember who he is when he died. Does that make sense? Overall, it's kind of a sad sentence.