Background: This paper describes the circumstances of contemporary young mothers and their children from a nationally representative sample, and compares them to the circumstances of mothers who delayed childbearing beyond age 20. Methods: The participants are members of the Environmental Risk (E-risk) Longitudinal Twin Study, which follows an epidemiological sample of 1,116 women who became mothers in England and Wales in 1994–95, and their children, and contains an over-sample of young mothers. Home visits were conducted when the children were aged 5 years. Data were collected from mothers via interviews, from children via experimental tasks and observations, and from teachers via postal questionnaires. Results: Young mothers encountered more socio-economic deprivation, had significantly less human and social capital, and experienced more mental health difficulties. Their partners were less reliable and supportive, both economically and emotionally, and were more antisocial and abusive. The children of young mothers showed reduced educational attainment, were rated by multiple informants as having more emotional and behavioural problems, were at increased risk of maltreatment or harm, and showed higher rates of illnesses, accidents, and injuries. Conclusions: Young mothers today face difficulties known to have long-lasting effects for women and their children. Preventions that target young mothers may reduce harm to the physical health, mental health, and social status of future generations.