Opinion Journal notes that there is similar confusion over the age of the new king, the Saudi Crown Prince, Fahd's brother Sultan, and indeed over the ages of all the royal siblings:
Precise birthdates are usually unknown, just years of birth. In itself that has potential for confusion because Ibn Saud, by virtue of having four wives at any one time, sometimes fathered as many as three sons in the course of a year. (Ibn Saud had 22 wives during his lifetime; births of daughters are even more imprecisely recorded but, for the purposes of this discussion, do not count.)As a result, Saudi princes can conveniently 'shave' years off to appear more fit for senior office -- or, if necessary, add them on to appear older, with more authority (and therefore, under different circumstances, also fit for senior office). The OJ author, a senior fellow of the Washington Institute for Near East Policy, says this is key to understanding Saudi Arabia over the coming years as all the candidates for kingship grow increasingly old and the House of Saud struggles to survive a series of short reigns.
I guess this murkiness leaves more options open as to who can succeed who -- both an advantage and a disadvantage. What fascinates me is the idea that in this day and age, members of a family with the Saudi royals' prominence -- the king himself! -- can still manage to obscure a basic detail like year (not even date...) of birth. I guess that's what it's like living in a real dictatorship -- a measure of just how tightly the Saudi royal family grips their country.