Who was the youngest pope of all times?

The youngest pope ever was John XII. According to the Catholic Encyclopedia, he was elected at the age of 18, while Philip Hughes says he was consecrated at the age of 16.
Pope John XII was almost certainly one of the worst popes who ever sat on the throne of Peter. His Father, Alberic, Prince and Senator of all the Romans, had used his influence to get some decent popes elected, but in an astounding break from tradition, just before he died, he made the Roman nobles promise they would elect his son, Octavius, as pope. Alberic had great aspirations for his family. He wanted to keep the papal states in the family, and to check the power of Otto I, Holy Roman Emperor. who aspired to rebuild Charlemagne’s Empire. When Alberic died, John became both the spiritual and temporal ruler of Rome.
John XII had little taste for religion. He loved hunting and fast women, many of whom were paid for through the church’s treasury.
According to many sources the youngest pope in history was Benedict IX. But this is just an urban myth.