“Early intervention” has been a mantra in recent debates about human capital investment. Strong theoretical models motivate this focus by predicting that investment in children is most cost-effective when they are young. The “Heckman curve” summarizes this idea visually (Heckman, 2006). However, hardly any reviews scrutinize this hypothesis empirically in modern welfare states such as those in Scandinavia that already invest heavily during early childhood. Any such review is ideally based on interventions conducted as randomized controlled trials (RCTs), set in the same welfare state, and comparable across ages through cost-standardized effects. This meta-analysis assembles cost-standardized effect estimates from 10 RCTs, including a total of 18 intervention arms and 30,578 participants (aged 1.5–24 years), conducted by the same research center in the Scandinavian welfare state of Denmark. These interventions show significant effects relative to their costs, despite the large baseline investment level. Interventions targeted at younger children tend to produce larger effects, consistent with the Heckman curve. However, variation in the effect size within age groups is as large as it is across age groups. This indicates that both the quality and timing of investments matter and that “early interventions” are not necessarily superior to later interventions.