Teenagers who go straight into the Army with no qualifications suffer twice as much stress-related mental illness as soldiers who have more education, according to a new report.
A study by ForcesWatch says younger troops from disadvantaged backgrounds are also more likely to drink heavily and behave violently after returning from war zones.
The human rights group is calling for the age of recruitment to be raised from 16 to 18 - pointing out the UK is the only country in the European Union to recruit soldiers at 16.
Its report says 8% of Iraq War veterans who enlisted without any GCSEs or equivalent qualifications have suffered post-traumatic stress disorder (PTSD), compared with 4% in the armed forces as a whole.
Just over a quarter of personnel aged 18-24 were found to be drinking heavily, which is twice the forces' average.
And over the past 20 years the suicide rate amongst male soldiers under 20 has been 82% higher than among civilians of the same age.
David Buck joined the Army aged 17. He served in Kosovo at the age of 19 and Iraq when he was 25.
He told Sky News he first suffered post-traumatic stress after returning from Kosovo, but didn't realise the symptoms until many years later.
"It takes about 13 years to manifest and then they come out in full flow," he explained.
"The symptoms were severe anxiety, hyper-sensitivity, alcoholism and the odd use of violence; not towards anybody, but holes in the wall, that kind of thing."
David received treatment through charities such as Combat Stress and is now working with Veterans For Peace.
"I didn't expect to see what I saw, nothing can prepare you for that. Being such a young age it is hard to make an informed decision.
"When I went (to Kosovo) I was a bit wet behind the ears, it was only when I got back that I realised how bad it was.
"If you join or deploy when you're a bit older then you've got a bit more life experience under your belt."
However, the Ministry of Defence said the report failed to highlight the benefits of life in the services for young people.
"(A career in the military) provides them challenging and constructive education, training and employment, equipping them with valuable and transferable skills for life," a spokesperson said.
"It is also important to put these figures in context as independent research shows the rates of PTSD are similar to rates in the civilian population and the rates of suicide are actually lower.
"However, we take this issue extremely seriously which is precisely why this Government has committed £7.4m to improving services and why we are working to reduce the stigma of mental illness through a number of initiatives and campaigns."
Last month, an exclusive Sky News report revealed that Reservist soldiers are twice as likely to suffer from mental illness as their regular counterparts.
