BACKGROUND The UK is the only permanent member of the UN Security Council that has a policy of recruiting 16 and 17 year old individuals into its regular Armed Forces. Little is known about the consequences of enlisting as a Junior Entrant (JE), although concerns have been expressed. We compare the mental health, deployment history, and pre-enlistment and post-enlistment experiences of personnel who had enlisted as JEs with personnel who joined as Standard Entrants (SEs). METHODS Participants from a large UK military cohort study completed a self-report questionnaire between 2014 and 2016 that included symptoms of probable post-traumatic stress disorder (PTSD), common mental disorders, alcohol consumption, physical symptoms and lifetime self-harm. Data from regular non-officer participants (n=4447) from all service branches were used in the analysis. JEs were defined as having enlisted before the age of 17.5 years. A subgroup analysis of participants who had joined or commenced adult service after April 2003 was carried out. RESULTS JEs were not more likely to deploy to Iraq or Afghanistan but were more likely to hold a combat role when they did (OR 1.25, 95% CI 1.00 to 1.56). There was no evidence of an increase in symptoms of common mental disorders, PTSD, multiple somatic symptoms (MSS), alcohol misuse or self-harm in JEs in the full sample, but there was an increase in alcohol misuse (OR 1.84, 95% CI 1.18 to 2.87), MSS (OR 1.51, 95% CI 1.04 to 2.20) and self-harm (OR 2.13, 95% CI 1.15 to 3.95) in JEs who had commenced adult service after April 2003. JEs remain in adult service for longer and do not have more difficulties when they leave service. CONCLUSIONS JEs do not have worse mental health than SEs, but there is uncertainty in relation to alcohol misuse, MSS and self-harm in more recent joiners. Monitoring these concerns is advisable.