This thesis departs from the observation that in some countries such as Canada the age gap in voter turnout between younger and older citizens is widening. It does so because of a rapid turnout decline among the youngest electoral cohorts. These findings prompt the following question: What are trends in the age gap in voter turnout between younger and older citizens in other Western democracies, and how can these trends be accounted for? Plotting over-time age differences for ten countries, this thesis shows that age patterns in voter turnout are far from generic. Evidence of a widening generational divide is found in Canada, Denmark, Great Britain, Norway, and the United States. The age gap in Finland, Germany, Italy, the Netherlands and Sweden, on the hand, turns out to be relatively stable or even trendless through time. Turning from description to explanation, this thesis presents a novel hypothesis to account for declining turnout levels among young voters. Combining the life-cycle and cohort/generation approaches to age differences in political participation, the later maturation hypothesis examines the idea that the timing of certain life-cycle events that are considered important for the development of political participation varies from one generation to the next. Since key events such as leaving school, starting a first job, getting married and starting a family nowadays take place at a higher average age than they used to, later maturation should be able to explain turnout decline among young voters at least partially. Based on data from the British Election Studies from 1964 to 2005, this thesis shows that delays in the timing of life-cycle events are indeed negatively related to individual level youth voter turnout. Attendance of religious services, levels of political interest, turnout at previous elections, perceived party differences and strength of party identification also explain over-time differences in youth voter turnout. The largest share of individual level youth turnout is, however, accounted for by turnout trends in the rest of the electorate. At the aggregate level later maturation is also shown to explain part of the widening of the age gap. The political-institutional context, however, explains between-country differences best.