Gustavus Adolphus of Sweden became king when he was 16, and inherited three wars. He then led his troops in battle (almost drowning once) and managed to end one of the wars with relatively minor loss (the Kalmar War against Denmark, where Sweden was at a severe disadvantage) by the time he was 18.

Of course, after 21, he earned his reputation as a military commander, so perhaps it was a sign of things to come.

Source: Jones, War in the Western World.

Edward III of England

He was crowned nominally at age 14 and began ruling effectively at age 17 after getting rid of his mother's consort Roger Mortimer.

He had lots of challenges to deal with, Scottish rebellions, Plagues, wars in France, plots etc. but he forged robust enough alliances in the early years of his reign that he was able to maintain the throne for fifty years.

Many of the major events of his life happened much later in his reign, but the fact that he was able to assert his power and sovereignty at such a young age made all the difference, he might not have survived otherwise.

Mehmet II of the Ottoman Empire conquered Constantinople when he was 21, and then continued to conquer massive swathes of the Balkans in his youth.

Queen Christina of Sweden became queen at the age of 6 and ruled for over 20 years, brokered peace deals, and did her best to try to end the 30 Year War. She finally abdicated for a lot of reasons including converting to Catholicism.

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