Young people were at the forefront of anticolonial struggles for their and their countries’ independence: but their roles, and the generational dynamics of decolonization, have rarely been acknowledged. Youth is often treated as a natural, age-bounded demographic, mobilised through assumed energy, volatility, or structural grievance, rather than as a historically produced classification that made certain forms of action and solidarity possible. Yet, it was through the language of “youth” and “young-ness” that colonial regimes and their opponents made claims about readiness and future, authorised or delegitimised collective action, and justified coercive and pedagogical interventions. Ignoring age therefore obscures a key mechanism through which both imperial control and anticolonial political subjectivities were made. This project responds to calls to take youth seriously. With a comparative emphasis of British and French empires across the twentieth-century Global South, we trace how “youth” became a politically intelligible category through the encounter of imperial social engineering and anti-colonial mobilisation. We argue that constructing “youth” was integral to empire-building, as colonial authorities governed populations through age-focused institutions and practices to produce “good” colonial subjects. At the same time, anticolonial actors co-opted this category of “youth” into a site of solidarity, built organisations and framed liberation as a generational struggle. In reconstructing these genealogies, the project reframes decolonisation as a negotiation over age, generation, and future, and provides conceptual tools for reading the continuing prominence of “youth” within contemporary political anxieties and mobilisations.