This study investigated the effects of flexibility HRM on employee outcomes over time, as well as the role of age in these relations. Based on work adjustment theory and AMO-theory, it was predicted that availability and use of flexibility HRM would be positively related to employee engagement, as well as higher job performance. Moreover, we postulated different hypotheses regarding the role of employee age. While generation theory predicts that younger generations would react more strongly to flexibility HRM in relation to engagement, SOC-theory of aging predicts that older workers respond more strongly in relation to job performance. A longitudinal study among US employees and a study among employees in 11 countries across the world showed that engagement mediated the relationships between availability of flexibility HRM and job performance. Moreover, we found partial support for the moderating role of age in the relations of flexibility HRM with the outcomes: flexibility HRM was important for younger workers to enhance engagement, while for older workers it enhanced their job performance. The study shows that the effectiveness of flexibility HRM depends upon employee age and the type of outcome involved, and consequently theory on flexibility at work should take the age of employees into account.Practitioners Points:1. Flexibility HRM can be used by organizations to enhance younger workers' engagement, while it can be used for older workers to enhance their job performance. 2. It is important for organizations to not only offer flexibility to their employees, but also to make sure that employees take advantage of these HR practices. 3. Flexibility HRM is important across the world, because it enables people across the world to balance demands from work as well as from private life.Keywords: Flexibility HRM, Engagement, Older Workers, Employee Age, SOC-Theory; Generation Differences 3 Many countries across the world face rapid demographic changes, such as the aging of the workforce, and the entrance of a new generation of employees, the so-called Generation Y (Twenge, Campbell, Hoffman, & Lance, 2010; United Nations, 2009). The baby boom generation (born between 1946 and 1965) is becoming older and birth rates have decreased, resulting in a workforce that will increasingly be composed of older workers and fewer younger workers.Therefore, organizations have to invest more effort in being attractive employers for younger as well as for older workers. It has been proposed that key to the retention of ageing workers is offering workplace flexibility (Hill et al., 2008). However, theory of workplace flexibility has not yet incorporated a lifespan perspective in the effects of flexibility, and therefore, the current study aims to bridge this theoretical and empirical gap in the literature.Workplace flexibility has traditionally been conceptualized as HR practices that help employees combine work and nonwork responsibilities, and in particular childcare (Allen, Johnson, Kiburz, & Shockley, 2013;Leslie, Manch...