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There were two main issues. The first was that the SWEDISH age of majority was 16. Queen Christina, daughter of Gustav Adolfus, was offered the crown at age 16, but by her own request, her coronation was delayed until she was nearly 18 (so she could finish her education). This was, of course, in opposition to the Norwegian statutes, which had the king's age of majority at 20.
The more pressing issue was the power struggle between the king's mother, the Duchess Ingeborg (herself a Norwegian), and other Norwegians, who steadily moved to curb her power.
http://en.wikipedia.org/wiki/Ingeborg_of_Norway
The idea of crowning her son king at age 15 (a year ahead of the Swedish practice), was a countermove on her part.