What's wrong with "Min søster var i flåden når hun var yngre."?
You use når in present tense, but in past tense you use da instead. Both words mean the same thing though, just the usage depends on the used tense.
It's not actually the same, is it? You can use 'når' in the past, but then this sentence would mean that she was in the navy whenever she was younger, as opposed to the times when she wasn't
"navy" also translates to "søværnet"
What is the official name for "The Navy" in Danish? Is it "flåden" or "marinen"?
It's actually "Søværnet". Both that and "marinen" should be put up as correct answers.