Abstract: The extensive, long-term consequences of childhood trauma on brain development and the resulting implications for military recruitment and service are the subject of a review submitted to the Australian Royal Commission into Defence and Veteran Suicide, which began in 2021. Evidence presented to the Defence Select Committee’s inquiry, Mental Health and the Armed Forces, Part One: The Scale of Mental Health Issues, published in 2018, has shown that the youngest recruits from socioeconomically disadvantaged backgrounds are the group in the British Armed Forces at greatest risk of mental health problems. The developing nervous system is more vulnerable than that of the adult to adverse and stressful events, and this has a knock-on effect on the susceptibility to mental health problems later in life, including post-traumatic stress disorder (PTSD). The UK is the only country in Europe and one of only a handful of countries worldwide that routinely recruits 16-year-olds into its Armed Forces, mostly the Army and particularly the infantry. These countries include no other member of the North Atlantic Treaty Organisation (NATO) and no other permanent member of the United Nations Security Council. As of 2011, 134 countries in the world had banned the practice. Currently, 26 countries recruit from the age of 17 years. Principal pre-military risk factors for mental health problems in service personnel include youth and those associated with socioeconomic disadvantage such as childhood adversity.