Can't do that on Viking .... its got a no one under 18 rule. As to the medallion. Ppl are overthinking this based on my 6 MC cruises. You can turn it into just a room key or take advantage of so many options. It's up to the individual. And no where is it written that you need to use it onshore.
As to DMW, I tried it once. Didn't fit my style and never went back. I eat when I'm hungry. I have no idea what time that will be tomorrow