Age-for-grade, a marker for school progression, is defined as the extent to which pupils are underage or overage for their grade. This thesis explores the causes and consequences of age-for-grade heterogeneity and its influences on school dropout and life transitions. Data for the analyses originate from a demographic surveillance site in a population of about 36,000 in Karonga district, northern Malawi. Linked surveys include data on socio-economic status, schooling, sexual behaviour, pregnancy and marriage. The first paper examines the effects of growth faltering (low height-for-age or stunting) in early (11-17 months) and late childhood (4-8years) on school outcomes (age at enrolment, age-for-grade at age 11 and grade repetition) to explore early causes of delayed enrolment and poor school progression. The main reason for being overage-for-grade is grade repetition. The second paper uses cross-sectional data on 8174 children in 2010, to examine the prevalence and risk factors (individual, household and school-level) for grade repetition in the following year. Using longitudinal data from 2007-2015, the third paper examines the relationship between age-for-grade and primary school dropout, with school completion as a competing event. The median age of dropout for girls is 19, with almost 90% still enrolled at age 15. Those overage were more likely to drop out of school than those on track, with girls having a higher rate of dropout than boys. The fourth paper shows that girls who were sexually active, as early as age 14, were five times more likely to drop out, while sexually active boys were twice as likely to drop out of school, compared to their sexually inactive peers. This was not explained by underlying poor school performance: the association with sexual debut and dropout was as strong among those on track in school as among those 3 or more years behind. In a companion paper, the opposite relationship is examined. Being out of school was strongly associated with increased rates of pregnancy, of sexual debut for girls not boys, and of marriage for girls and boys. Age-for-grade as early as age 10 predicted age of pregnancy and marriage.