My friend had her baby at 38 and the NT screening test also came back as possible downs. Her DD is a perfectly healthy six year old now. Try not to panic, everything is probably just fine especially if your donor is only 21. It is only a screening test that shows the possibility of downs. I'm sorry you keep running into road bumps
Did they explain CVS testing to you? I think that is the next test you would take to rule out any downs.