Gerontocracy, in its narrowest sense, refers to political systems ruled by elderly people, whether de jure or de facto. Although formal gerontocratic rules are progressively disappearing, contemporary political systems are still governed by individuals who are significantly older than the mean voter. This article reviews existing explanations for the prevalence of gerontocracy. To summarize main findings, gerontocracy cannot be explained by the leadership qualities of older rulers: aging leaders do not perform better in office and voters seem to be aware of it. Instead, existing research suggests that gerontocracy can be explained by strategic considerations. In autocracies, the selectorate tends to choose aging leaders in order to reduce their expected tenure length. In democracies, voters are more likely to select experienced candidates, which they expect to be more effective at advancing the interests of their constituency: this premium put on experience mechanically lengthens political careers and increases the age of the average politician. Finally, older voters, which participate more in politics, tend to prefer older politicians, because they (correctly) expect them to better defend their own interests.