It is now known that the onset of severe and recurring mental health problems begins for the most part before the age of 25: this provides a clear focus for preventive strategies and public mental health that are a feature of many health policy frameworks. The present distinction between child and adolescent mental health services and adult services at 16 or 18 does not fit easily with these data and the now well-documented problems of transition suggest that a fundamental review of services for young people is overdue. This supplement provides an overview of the epidemiological, conceptual and service structures for young people with emergent and existing mental health problems, and asks the question, ‘How should we design services for young people to promote prevention and service engagement, and to improve outcomes?’

Although the corrosive effect of mental ill health on human health and happiness has long been recognised, it is only relatively recently that mental illness has been acknowledged as one of the major threats to economic productivity worldwide. This is because the major mental disorders most commonly have their onset during adolescence and early adulthood, and therefore have a disproportionate impact on the most productive decades of life. With the costs associated with mental ill health estimated to double over the next two decades, a greater emphasis on prevention and early intervention has become even more imperative. Although prevention largely remains aspirational for many reasons, early intervention is well within our current reach and offers the potential to significantly reduce the impact of mental ill health on our health, happiness and prosperity in the immediate future.

The study of age at onset of mental health disorders is technically and conceptually difficult. It is important to consider these age distributions in order to understand causes and mechanisms of illness and to intervene at an appropriate juncture for primary and secondary prevention. This article reviews some of the approaches to studying age at onset, sets out the evidence to support the assertion that adult mental disorders begin in adolescence, and finds that perhaps half of all adult mental health disorders have begun by the teenage years. The paper then discusses whether this fits what is known about the developmental neurobiology of the brain and introduces the implications for mental health services.

A new approach to understanding severe mental illnesses such as schizophrenia and affective disorders is to adopt a clinical staging model. Such a model defines the extent of the illness such that earlier and milder phenomena are distinguished from later, more impairing features. Part of the appeal of such a model is that it should have cross-diagnostic applications, but to date there has been no attempt to examine imaging or neurocognrtive evidence for staging in this way. We review these two domains of study with particular focus on major depression and bipolar affective disorder. Although there is some support for the staging model in affective disorders, conclusions are limited by the large variability in the clinical samples studied, especially with regard to the presence of psychotic symptoms. We suggest that future research needs to take a transdiagnostic and longitudinal approach.

Borderline personality disorder (BPD) is a leading candidate for developing empirically based prevention and early intervention programmes because it is common in clinical practice, it is among the most functionally disabling of all mental disorders, it is often associated with help-seeking, and it has been shown to respond to intervention, even in those with established disorder. Moreover, it can be reliably diagnosed in its early stages and it demarcates a group with high levels of current and future morbidity and mortality. Data also suggest considerable flexibility and malleability of BPD traits in youth, making this a key developmental period during which to intervene. Novel indicated prevention and early intervention programmes have shown that BPD in young people responds to intervention. Further work is required to develop appropriate universal and selective preventive interventions.

Despite the evidence showing that young people aged 12-25 years have the highest incidence and prevalence of mental illness across the lifespan, and bear a disproportionate share of the burden of disease associated with mental disorder, their access to mental health services is the poorest of all age groups. A major factor contributing to this poor access is the current design of our mental healthcare system, which is manifestly inadequate for the unique developmental and cultural needs of our young people, if we are to reduce the impact of mental disorder on this most vulnerable population group, transformational change and service redesign is necessary. Here, we present three recent and rapidly evolving service structures from Australia, Ireland and the UK that have each worked within their respective healthcare contexts to reorient existing services to provide youth-specific, evidence-based mental healthcare that is both accessible and acceptable to young people.

This discussion paper outlines our personal views for debate on some of the complexities inherent in the crucial task of improving mental health services for young people in the UK.