According to the Wikipedia article Magnus IV of Sweden
Magnus was declared to have come of age at 15 in 1331. This caused resistance in Norway, where a statute from 1302 made clear that kings came of age at the age of 20, and a rising by Erling Vidkunsson and other Norwegian nobles ensued. In 1333, the rebels submitted to king Magnus.
Who declared him to come of age and why? Was it his own idea? Why couldn't they wait for 5 more years? Which enemies were they afraid of?