This study examines trends in first marriage, consensual unions, and cohabiting unions among adolescents. Data were obtained for 40 countries from Demographic and Health Surveys. Marriage timing was grouped for the proportion of all women aged 20-24 years who married by ages 15, 18, and 20 years. Women were most likely to marry at young ages in sub-Saharan Africa. 60-92% of women aged 20-24 years married by age 20. High proportions of early marriage were found in Bangladesh, Guatemala, India, and Yemen. In developing countries other than sub-Saharan African countries, marriages before the age of 18 years ranged between 20% and 33%. Marriages before the age of 20 years ranged between 33% and 50%. In developed countries such as France or the US, first marriage by the age of 18 years among 20-24 years olds represented 11% of all first marriages. Japan was one of the few countries with a very low proportion of early marriages: 2% of 20-24 year olds married before the age of 20 years. Marriage before the age of 18 years was uncommon in Botswana, Namibia, the Philippines, Sri Lanka, and Tunisia and comparable to US and French proportions. Marriage before the age of 15 years was common only in Bangladesh and Niger (50%). Very early marriage ranged between 10% and 27% in Cameroon, Liberia, Mali, Nigeria, Senegal, Togo, Uganda, Guatemala, India, Indonesia, Pakistan, Sudan, and Yemen. The median age at first marriage ranged widely within and across regions. Over time the proportion married before the age of 20 years declined for cohorts aged 20-24 years, 30-34 years, and 40-44 years. The magnitude of decline in the proportion married before age 20 varied substantially across regions. Increases were larger and more widespread in North Africa, the Middle East, and Asia. Later age at marriage was more prevalent among urban women, in countries with greater urbanization, and among women with at least a secondary education in urban areas.