> However, the hospital refused to test them because they were "young and healthy" (i.e. no underlying medical conditions to complicate things).

The tests have a high false negative rate, which is one reason some countries are not testing everyone with mild symptoms. The test doesn't give you any useful information in that situation: if it comes back positive you need to self-isolate, but if it comes back negative and you have symptoms you still need to self isolate.