A point that will be repeatedly made about the death of Saudi King Abdullah and the transfer of power to his "younger" brother, 79-year-old Salman, is that the Saudi ruling family is very old. The new king is so old that his father, King Abdulaziz, was born in 1876, when the US only had 38 states. That's not just fun trivia: the agedness of the Saudi monarchy is dangerous.
The danger is that, when a country's new king is already old enough to be a dozen Caribbean cruises deep into retirement, that country is setting itself up for instability. Saudi Arabia is not well equipped to handle political instability, and the world, ever reliant on Saudi oil, is not well equipped to handle it either.
There are two ways in which the agedness of the Saudi royal family is dangerous. The first kind is that Salman could die or become senile early in office, and so could his successor, the already very old Muqrin. Even if those individual transitions go okay, a rapid series of unexpected successions could destabilize a rickety and secretive political system that is not build to handle shocks well.
The second kind is that the new crown prince, the 69-year-old Muqrin, is the last of this generation of Saudi male heirs, and the next generation has many dozens of possible heirs who will almost certainly fight among each other over succession, setting up the remote but real possibility of a succession crisis or worse.
It is worth emphasizing that monarchies in general, and this one in particular, are inherently unstable. A democracy like America's can change out leaders every four years and be fine because power is built on institutions and laws that always stay the same. In a monarchy like Saudi Arabia's, power is built on personal relationships and loyalties, which are opaque and can shift easily. That's part of why most absolute monarchies in the world have long ago collapsed.
The risk is especially high in Saudi Arabia, where the government has managed to halt the tide of Arab Spring-style popular unrest by handing out lavish oil-funded social services, and of course by old-school, medieval-style repression. Maybe the coming transitions and succession battles will only cause minor shocks in the Saudi political system, but it's just very hard to tell from the outside if the Saudi system can endure even that.