They include the following factors
-Longevity: years the regime was in power (15% of index)
-Youth: % of population under 25
-GDP per capita (purchase parity)
-Freedom of Press
Longevity: It does seem that longevity increases instability. This is counter-intuitive as traditionally long lasting leaders are more immune to change (see how several countries, notably in Africa in past decades, had coup d'etats every couple years, whereas in other cases a leader would hold on for thirty years). My guess is that this could be a new type of movement, so the source of stability (repression/wealth/corruption) actually works against them. Or it could be that we're just seeing a small sample size and the factor doesn't matter. Or it could be that leader age is the real problem - old leaders are out of touch and lack the energy/ability to change.
Youth: I agree. This is a critical factor. Young people are more likely to lack fear and revolt. They didn't include youth unemployment.
Total Population: this should not be a factor. No idea why they included it.
GDP per capita: I've got mixed feelings about this. People who are well off are generally more content (and I agree that governments can buy them off) and might be less likely to take risks, but having greater availability of technology, education, and travel can encourage revolution. I think having a change in GDP growth would be significant (so if an economy was growing and educating its young people but then stopped growing and they had few opportunities)
In general I think they are lacking some key variables including Al-Jazeera viewership, age of leader, youth unemployment, gdp growth over the past 1-2 years, internet access, mobile phone access, and level of education.