Question: What was the age of Magnus Erikson when he mounted the Norwegian throne under the name of King Magnus VII?
About the article "Norway", section "Kalmar Union".

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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. 
Accession to the throne.
Sweyn was born in England, as the son of Norwegian Ulf Jarl and Estrid Svendsdatter, the daughter of king Sweyn I of Denmark and sister of king Canute the Great.
Sweyn grew up a military leader, and served under king Anund Jacob of Sweden for a time.
He pillaged the Elbe-Weser area in 1040, but was caught by the Archbishop of Hamburg-Bremen, who released him shortly thereafter.
Svend was made "jarl" under Danish king Harthacnut, and led a campaign for him against Norway, but was beaten by Magnus I of Norway.
When Harthacnut died in 1042, Magnus claimed the Danish throne and made Svend his "jarl" of Jutland.
Sweyn fought for Magnus at the Battle of Lyrskov Heath in 1043.
Sweyn won great reputation at Lyrskov Heath, and had the Danish nobles crown him king in Viborg in Jutland.
He was defeated by Magnus on several occasions, and had to flee to Sweden.
Eventually he managed to return and establish a foothold in Scania.
The war between Magnus and Sweyn lasted until 1045, when Magnus' uncle Harald Hardrada returned to Norway from exile.
Harald and Sweyn joined forces, and Magnus decided to share the Norwegian throne with Harald.
In 1047 Magnus died, having stated on his deathbed that his kingdom would be divided: Harald would get the throne of Norway, while Sweyn would be king of Denmark.
Upon hearing of Magnus' death Sweyn said, "Now so help me God, I shall never yield Denmark."
Magnus was proclaimed king in 1035, at 11 years of age, and Svein and his mother fled; Svein died shortly after.
At first Magnus sought revenge against his father's enemies, but on Sigvatr's advice he stopped doing so, which is why he became known as "good" or "noble".
Another son of Cnut, Harthacnut, was on the throne of Denmark and wanted his country to reunite with Norway, while Magnus initiated a campaign against Denmark around 1040.
However, the noblemen of both countries brought the two kings together at the Göta River, the border between their kingdoms.
They made peace and agreed that the first of them to die would be succeeded by the other.
In 1042 Harthacnut died while in England, and Magnus also became King of Denmark, in spite of a claim by Cnut's nephew Sweyn Estridsen, whom Harthacnut had left in control of Denmark when he went to England, and who had some support.
As part of consolidating his control, Magnus destroyed the Jomsborg, headquarters of the Jomsvikings.
Sweyn fled east and returned as one of the leaders of an invasion by the Wends in 1043, which Magnus decisively defeated at the Battle of Lyrskov Heath, near Hedeby.
In the battle, Magnus wielded Saint Olaf's battle-axe, named Hel after the goddess of death.
He had dreamt of his father the night before, and the Norwegians swore that before the battle they could hear the bell that Saint Olaf had given to the Church of St. Clement in Kaupang, in Nidaros - a sign that the saint was watching over his son and the army.
It was the greatest victory ever over the Wends, with up to 15,000 killed.
Sweyn continued to oppose Magnus in Denmark, although according to "Heimskringla", they reached a settlement by which Sweyn became Earl of Denmark under Magnus.
Magnus wanted to reunite Cnut the Great's entire North Sea Empire by also becoming king of England.
When Harthacnut died, the English nobles had chosen as their king Æthelred the Unready's son Edward (later known as Edward the Confessor); Magnus wrote to him that he intended to attack England with combined Norwegian and Danish forces and "he will then govern it who wins the victory."
The English were mostly hostile to Magnus; Sweyn was made welcome there, although Edward's mother, Emma, curiously favored Magnus and in 1043 the king confiscated her property, with which by one report she had promised to assist Magnus.
Meanwhile, Magnus' uncle Harald Hardrada had returned to Norway from the east and contested his rule there, while Sweyn was still a threat in Denmark; Harald allied himself with Sweyn.
Magnus chose to appease Harald, and made him his co-king in Norway in 1046. 
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. 
In 1319 the infant Magnus Henriksson was crowned as king of both Sweden and Norway.
In 1332 when the King of Denmark Christopher II died as a "king without a country" after he had pawned Denmark piece by piece, King Magnus took advantage of his neighbour's distress, redeeming the pawn for the eastern Danish provinces for a huge amount of silver, and thus also became king of Skåneland.
The union of these three countries lasted until 1343 when Magnus preemptively let his son Haakon, succeed him to the Norwegian throne, though he would still rule as regent during his son's minority, which ended in 1355, when Haakon came of age.
In 1360 the Danish king Valdemar Atterdag reconquered Skåneland.
The Swedish kingdom at this time included Skåneland (1332-1360), Sweden and Finland, while the Norwegian kingdom included Greenland, Iceland, and the Faroe, Shetland and Orkney Islands.