There is a common assumption in scientific and policy discourse that, for greatest effect, interventions need to be applied early in life, when children’s brain function and behaviour are thought to be more malleable. Surprisingly, very few studies have tested directly whether common interventions for child development, mental health and behaviour are more effective when delivered earlier, rather than later in childhood. We assess this question using data from multiple trials of parenting interventions in Europe, and updated reviews from around the world, drawing on both individual participant (IPD) and aggregate level meta-analytic approaches. Implications for other fields of child development, and for policy will be discussed.