Justice Across Ages is a book about how we should respond to inequalities between people at different stages of their lives. Age structures our social institutions, relationships, obligations, and entitlements. There is an age for voting, an age for working, and an age when one is expected (and sometimes required) to retire. Each stage of life also corresponds to specific forms of social risks and vulnerabilities. As a result, inequalities between age groups and generations are numerous and multidimensional. And yet, political theorists have spared little time thinking about how we should respond to these disparities. Are they akin to those patterned on gender or race? Or is there something relevantly distinctive about them that mitigates the need for concern? These questions and others are answered in this book and a theory of justice between co-existing generations is proposed. Age structures our lives and societies. It shapes social institutions, roles, and relationships, as well as how we assign obligations and entitlements within them. There is an age for schooling, an age for voting, an age for working, and an age when one is expected (and sometimes required) to retire. Each life-stage also brings its characteristic opportunities and vulnerabilities, which spawn multidimensional inequalities between young and old. How should we respond to these age-related inequalities? Are they unfair in the same way that gender or racial inequalities often are? Or is there something distinctive about age that should mitigate ethical concern? Justice Across Ages addresses these and related questions, offering an ambitious theory of justice between age groups. Written at the intersection of philosophy and public policy, the book sets forth ethical principles to guide a fair distribution of goods like jobs, healthcare, income, and political power among persons at different stages of their life. Drawing on a range of practical cases, the book deploys normative tools to distinguish objectionable instances of inequalities from acceptable ones and in so doing, critically assesses a range of policy remedies. At a time where young people are starkly under-represented in legislatures and subject to disproportionally high unemployment rates, the book moves from foundational theory to the specific policy reforms needed today. As moral and political philosophers have noted, it can be tempting to assume that age-based inequalities are morally trouble free, since over the course of a complete life, a person moves through each age groups. Yet, Justice Across Ages argues that we should resist this assumption. In particular, we should regard with suspicion commonplace and widely tolerated forms of age-based social hierarchy, such as the infantilization of young adults and older citizens, the political marginalization of teenagers and young adults, the exploitation of young workers through precarious contracts and unpaid internships, and the spatial segregation of elderly persons. If we ever are to live in a society where people are treated as equals, we must pay vigilant attention to how age membership can alter our social standing. This position carries important implications for how we should think about the political and moral value of equality, design our social and political institutions, and conduct ourselves in a range of contexts that includes families, workplaces, and schools.