In England and Wales, the academic year begins in the September, and children start school in September before they are five years old. If children are born in September, then they are nearly five when they start school, but if they are born in August in the following chronological year then they have only just turned four years old when the school year starts. Studies have shown that the youngest children within a school year are at an increased risk for mental health problems, social impairment, neurodevelopmental disorder and intellectual disability diagnoses, and lower educational attainment (Bedard & Dhuey, 2006; Zoëga et al., 2012; Pottegård et al, 2014; Root et al., 2019). Cross-national comparisons of large representative population surveys that compare countries with different school entry dates have suggested that associations may reflect causal influences of age within school year on these outcomes, rather than season-of-birth (Goodman et al., 2003). This project will focus on children with early neurodevelopmental vulnerability, which will be defined using neurodevelopmental symptoms and diagnoses, genetic risk, or prematurity of birth. These children are all already at a higher risk of mental health problems including depression (Rice et al., 2018). We hypothesise that relative age effects may affect these groups of children more than others over development from childhood to adulthood. We also hypothesise that differences in mental health by month of birth will emerge only after school entry but show some persistence across the school years into early adulthood.