A lot of Norwegians don't distinguish 'da' and 'når', and only use 'når', which has been the bane of many Norwegian teachers. They might merge in the future.
'da' is used when it has happened in the past. "Da jeg var liten" = "When I was young".
'når' is about the future/repeating events. "Når jeg blir gammel" = "When I get old"(future) "Når jeg fisker" = "When I'm fishing"(repeating).