I’ve drawn Denmark as younger than Sweden and Norway a few times, but that had to do with the age of the landmass. This shows how old the Nordics are as countries.
Denmark’s birth is from when it became a united country instead of tribes fighting each other, Sweden is from King Gustav Vasa's crowning and Norway, Finland and Iceland are from when they became fully independent from all other countries.
Norway is a bit tricky though, because he could also be said to be around the same age as Denmark. But this is how it's usually done.