Question: How old was Magnus Erikson when he inherited the throne as King Magnus VII of Norway?
About the article "Norway", section "Kalmar Union".

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Upon the death of Haakon V, King of Norway, in 1319, Magnus Erikson, at just three years old, inherited the throne as King Magnus VII of Norway.
At the same time a movement to make Magnus King of Sweden proved successful.
(At this time both the kings of Sweden and of Denmark were elected to the throne by their respective nobles.)
Thus, with his election to the throne of Sweden, both Sweden and Norway were united under King Magnus VII.
In 1349, the Black Death radically altered Norway, killing between 50% and 60% of its population and leaving it in a period of social and economic decline.
The plague left Norway very poor.
Although the death rate was comparable with the rest of Europe, economic recovery took much longer because of the small, scattered population.
Before the plague, the population was only about 500,000 people.
After the plague, many farms lay idle while the population slowly increased.
The few surviving farms' tenants found their bargaining positions with their landlords greatly strengthened.
King Magnus VII ruled Norway until 1350, when his son, Haakon, was placed on the throne as Haakon VI.
In 1363, Haakon VI married Margaret, the daughter of King Valdemar IV of Denmark.
Upon the death of Haakon VI, in 1379, his son, Olaf IV, was only 10 years old.
Olaf had already been elected to the throne of Denmark on 3 May 1376.
Thus, upon Olaf's accession to the throne of Norway, Denmark and Norway entered personal union.
Olaf's mother and Haakon's widow, Queen Margaret, managed the foreign affairs of Denmark and Norway during the minority of Olaf IV.
Margaret was working toward a union of Sweden with Denmark and Norway by having Olaf elected to the Swedish throne.
She was on the verge of achieving this goal when Olaf IV suddenly died.
However, Denmark made Margaret temporary ruler upon the death of Olaf.
On 2 February 1388 Norway followed suit and crowned Margaret.
Queen Margaret knew that her power would be more secure if she were able to find a king to rule in her place.
She settled on Eric of Pomerania, grandson of her sister.
Thus at an all-Scandinavian meeting held at Kalmar, Erik of Pomerania was crowned king of all three Scandinavian countries.
Thus, royal politics resulted in personal unions between the Nordic countries, eventually bringing the thrones of Norway, Denmark, and Sweden under the control of Queen Margaret when the country entered into the Kalmar Union.
Magnus IV (April or May 1316  – 1 December 1374; Swedish "Magnus Eriksson") was King of Sweden (including Finland) from 1319 to 1364, King of Norway as Magnus VII (including Iceland and Greenland) from 1319 to 1343, and ruler of Scania from 1332 to 1360.
By adversaries he has been called "Magnus Smek" (English: "Magnus the Caresser").
Referring to Magnus Eriksson as "Magnus II" is inaccurate.
The Swedish Royal Court officially lists three Swedish kings before him by the name. 
Magnus Olafsson (Old Norse: "Magnús Óláfsson", Norwegian and Danish: "Magnus Olavsson"; c. 1024 – 25 October 1047), better known as Magnus the Good (Old Norse: "Magnús góði", Norwegian and Danish: "Magnus den gode"), was the King of Norway from 1035 and King of Denmark from 1042, ruling over both countries until his death in 1047.
He was an illegitimate son of Olaf II of Norway, but fled with his mother when his father was dethroned in 1028.
He returned to Norway in 1035 and was crowned king at the age of 11.
In 1042, he was also crowned king of Denmark.
Magnus ruled the two countries until 1047, when he died under unclear circumstances.
After his death, his kingdom was split between Harald Hardrada in Norway and Sweyn Estridsson in Denmark. 
Olav Ugjæva (Old Norse: "Ólafr úgæfa") (died 1169) was a pretender to the Norwegian throne during the civil war era in Norway.
Olaf was named king in 1166, but was subsequently defeated by King Magnus V of Norway ("Magnus Erlingsson") and forced to flee the country. 
Establishing authority.
Norway had experienced a long period of peace during the reign of Magnus' father, Olaf.
Magnus may have been present when Olaf died in Ranrike, Båhuslen (southeastern Norway) in September 1093 and was probably proclaimed king at the Borgarting, the thing (assembly) of the adjacent region of Viken later that month.
When Magnus became king, he already had a network of support among the Norwegian aristocracy.
Although sources are unclear about the first year of his reign, it is apparent that Magnus' focus was on the west (towards the British Isles).
Since conditions were chaotic in Norse-dominated parts of the British Isles since the death of Thorfinn the Mighty, this provided Magnus an opportunity to intervene in local power struggles.
According to some accounts, he made his first expedition west in 1093–94 (or 1091–92), helping Scottish king Donald Bane to conquer Edinburgh and the Scottish throne and possibly gaining control of the Southern Isles ("Suðreyjar") in return.
It is unclear if this early expedition took place, since it is not directly referenced in early reliable sources or the sagas.
Magnus was opposed by his cousin Haakon Magnusson, son of King Olaf's brother and short-lived co-ruler King Magnus Haraldsson, who claimed half the kingdom.
Haakon was proclaimed king in the Uplands and at the Øyrating, the thing of Trøndelag (in central Norway).
According to Førsund, Haakon took control of the entire portion of the kingdom once held by his father (also including the Frostating—the thing of Hålogaland in northern Norway—and the Gulating—the thing of western Norway).
Haakon secured support by relieving farmers of taxes and duties (including taxes dating back to the Danish rule of Sweyn Knutsson during the early 1030s), while Magnus pursued costly policies and demanded lengthy military service.
After Magnus settled at the new royal estate in Nidaros for the winter of 1094–95, Haakon also travelled to the city and took up residence at the old royal estate.
Their relationship became increasingly tense, culminating when Haakon saw Magnus' longships fully rigged at sea.
Haakon summoned the Øyrating in response, leading Magnus to sail southwards.
Haakon attempted to intercept Magnus by travelling south to Viken by land (over the mountains of Dovrefjell), but he died unexpectedly while hunting in February 1095.
The strongman behind Haakon's monarchy had been his foster-father Tore Tordsson ("Steigar-Tore"), who refused to recognise Magnus as king after Haakon's death.
With Egil Aslaksson and other noblemen, he had the otherwise-unknown Sweyn Haraldsson set up as a pretender.
Although later sagas maintain that Sweyn was Danish, some modern historians have speculated that he may have been a son of Harald Hardrada.
The revolt was based in the Uplands, but also gained support from noblemen elsewhere in the country.
After several weeks of fighting, Magnus captured Tore and his supporters and had them hanged on the island of Vambarholm (outside Hamnøy, Lofoten, in northern Norway).
Magnus was reportedly furious because he could not pardon Egil, a potentially useful, young and resourceful nobleman.
As king, his honour would only allow a pardon if other noblemen pleaded for Egil's life; this did not happen.
Magnus' final domestic dispute was with the noble Sveinke Steinarsson, who refused to recognise him as king.
Although Sveinke reduced piracy in Viken, he was forced into exile for three years after negotiating with Magnus' men.
Since piracy increased soon after Sveinke's departure (possibly encouraged by Sveinke himself), Magnus met him in the Danish province of Halland to request his return to Norway.
They reconciled; Sveinke became a loyal supporter of Magnus, now the undisputed king of Norway.