Studying oppressive autocracies around the world in the period 1972 to 1989, he found that if their median population age was over 35, they were unlikely to be subject to revolution at all. Of those that experienced revolutions when their median age was between 30 and 35, all are still democracies today. Ninety per cent of those with a median age less than 25 ultimately reverted back to oppressive regimes. With median ages between 25 and 30, they could go either way.
So what does this mean for the Arab Spring countries? Well, Tunisia, with a median age of 30, is the most likely to transition to a permanent democracy. With median ages of 25 and 26 respectively, Egypt and Libya have a chance, but for Syria and Yemen, at 21 and 17, the odds are very poor indeed.
As for the country we are most invested in—Afghanistan—with a median age of 18, a permanent transition to democracy may be a long time coming.
Although age appears to be a useful indicator for stable transitions, it is not likely a cause. That is more likely reflected by the maturity of a country. More mature, complex societies—urbanized, high income, greater gender equality, better educated, etc.—are more equipped for democracy and also have lower birth rates and therefore older populations.
Older generations often say that youth is wasted on the young. In the case of revolution, it would seem to bear a kernel of truth.