In the 1880 Denmark Census Marius Mortensen is listed in the house of Jørgen and Karen Mortensen. Marius is listed a Deres Søn age 11; which would put his birth at about 1869. One might ask how is this possible, Jørgen and Karen were not married until 1872. Well if we go to the 1870 Denmark Census we can shed a little bit of light on the situation. In the 1870 there is a house with the following people listed:
Rasmus Mortensen - Widow - age 39 - Huusfader Slagter
Lisbet Mortensen - age 8 - Datter
Søren Søberg Mortensen - age 6 - Søn
Rasmine Mortensen - age 3 - Datter
Rasmus Marius Mortensen - age 1 - Søn
Jørgen Mortensen - Single - age 36 - Broder Slagtersvend
Søren Mortensen - Single - age 19 - Brodersøn Slagtersvend
So now we know that Marius is the son of Rasmus Mortensen, not Jørgen Mortensen. If we do a little bit more digging we find that Rasmus Marius Mortensen was born December 20, 1868 to Rasmus Mortensen and Ane Marie Pedersen. Digging a little bit deeper we find out that Ane Marie Mortensen died October 29, 1869 of Skarlagensfeber.
So know we know the story of of Marius Mortensen. The assumption is that Jørgen and Karen adopted him and that is why he is listed as Deres Søn.
Danish to English:
Huusfader = Head of Household
Datter = Daughter
Søn = Son
Deres Søn = Their Son
Broder = Brother
Brodersøn = Nephew
Slagter = Butcher
Slagtersvend = Butcher's Assistant
Skarlagensfeber = Scarlet Fever