We use Grounded Theory to study the evolution of requirements practices of 16 software startups as they grow and introduce new products and services. These startups operate in a dynamic environment, with significant time and market pressure, and rarely have time for systematic requirements analysis. Our theory describes the evolution of practice along six dimensions that emerged as relevant to their requirements activities: requirements artefacts, knowledge management, requirements-related roles, planning, technical debt and product quality. Beyond the relationships among the dimensions, our theory also explains the turning points that drove the evolution along these dimensions. These changes are reactive, rather than planned, suggesting an overall pragmatic lightness, i.e., flexibility, in the startups’ evolution towards engineering practices for requirements. Our theory organises knowledge about evolving requirements practice in maturing startups, and provides practical insights for startups’ assessing their own evolution as they face challenges to their growth. Our research also suggests that a startup’s evolution along the six dimensions is not fundamental to its success, but has significant effects on their product, their employees and the company.