At just 32, Muhammad bin Salman (MBS) owns the world's most expensive home - a $300m chateau in France which was built in the style of Louis XIV. He owns a yacht worth $500 million and a $450 million Leonardo da Vinci painting. And now it appears he owns Saudi Arabia too. When King Salman ascended to the Saudi throne in 2015, few had heard of his ambitious young son. But in June 2017 MBS won a power struggle against his cousin and was appointed Crown Prince - the heir to the throne in all but name. Then, in November, he struck, rounding up his opposition. Over one weekend, Saudi police arrested 11 princes, and dozens of businessmen and government officials. MBS and his father tried to paint the purge as a drive against corruption. But according to US foreign policy specialist Colin Kahl, "Corruption charges can be generated on just about anyone. This looks like the final step to consolidate MBS's authority by removing challengers." The House of Saud's succession rules have resulted in the country becoming an gerontocracy. Power passes from brother to brother, rather than from father to son, meaning that the king is almost always very old. King Salman is 81 and is rumoured to have dementia. The country's ultra-conservative religious elite is similarly grey-haired. This is what makes MBS so different. Around 70% of the Saudi population is under 30, MBS is relying on their support as he aims to push through a raft of reforms to open the forbidden kingdom up. He forced through a new law allowing women to drive. He wants to return the country to "moderate Islam". He plans to open cinemas and welcome tourists. He has banned the country's religious police from arresting people. But his aggressive foreign policy leaves many sceptical of his charms: he has been the main driving force behind Saudi Arabia's war in Yemen, and his loathing of Iran could trigger "an Islamic civil war". Should he really be hailed as "person of the year"? "The prince's liberalising rhetoric cloaks far more sinister motives", writes Rosie Bsheer in The Washington Post. How has the world been duped by a man who arrests his cousins, forces foreign leaders to resign and starts aggressive wars? Saudi Arabia has been stable for years. We should not cheer anyone who threatens that. But others argue that the kingdom is far from stable. One piece in The Atlantic labels it "an unsustainable entity so corrupt as to resemble a criminal organisation". The rise of MBS and the change that will bring may prevent a disaster which would shake the world. He may be a morally dubious child of fortune, but history tells us that the most important reformers are rarely complete outsiders.