Soldiers who joined the army before they were 18 are significantly more likely to suffer from post-traumatic stress disorder (PTSD) and other serious mental health problems when leaving the forces, according to a report published on Monday.

Younger recruits also have a higher risk of alcohol problems, depression and suicide than those who signed up as adults, claims the report, The Last Ambush, from ForcesWatch, which campaigns for ethical recruitment in the armed forces.

Britain is one of just 19 countries that recruit 16-year-olds into the army. Zimbabwe recently increased its minimum soldiering age to 18.

Under-18s are over-represented in the infantry – the report says that over the past five years 32% of all under-18s recruited joined the infantry, which makes up only 14% of Britain's armed forces. Recruits cannot be deployed to the frontline until they turn 18.

David Buck joined the army at 17, saw active duty in Kosovo when he was just 19, and witnessed mass graves and burning bodies. On returning to civilian life at 26 he was diagnosed with PTSD, which he attributes to seeing such horrific images at such a young age. He also experienced bouts of severe alcoholism when he returned from fighting in Iraq. "I was trying to get away from the mental torture of PTSD," he told the Guardian.

Buck says he was swayed by the brochures he read at the recruitment office. "It's just deception. It doesn't show someone with their head blown off." He recalls images that glamorised army life, with recruits abseiling and skiing. "Being so young I was easily manipulated with the stuff they shovel down your neck in the careers office," he said.

Richard Pendleton, a marketing executive who worked on army recruitment campaigns for 10 years, has told the Guardian that he believes the material he created was misleading.

"I'm deeply uncomfortable about the way we sold the army to young people." He claims he was expected to avoid mentioning the negatives of army life, adding: "You're never using words like 'kill' or 'maim' – they would put people off."

The ForcesWatch report claims that younger recruits who experienced war were more likely to display post-deployment symptoms including PTSD. Most of the mental health-related problems in the report – anxiety and depression, harmful drinking, violence, self-harm and suicide – are more likely to occur the younger the soldier joined.

Rachel Taylor from Child Soldiers international said: "The UK is the only country in Europe and the only permanent member of the [UN] security council that persists in recruiting children into their armed services." Since 2002, the UN Committee on the Rights of the Child, the Commons defence select committee and the joint committee on human rights have all repeatedly called on the Ministry of Defence to review its policy of recruiting under-18s. The MoD has refused to conduct a review.

Under-18s made up 23.4% of army intake last year, and several former soldiers are speaking out against recruiting minors. Chris Slater, 50, first walked into a Newcastle recruitment office when he was 14 years old. He was encouraged to join the Royal Engineers and signed up before he was 15.

He claims he later learned that recruitment officers earn commission on the teenagers they sign up, and believes he was "indoctrinated" when he was too young to make meaningful life choices.

This view is echoed by Pendleton, who initially believed that the glossy magazines promising a fun and exciting life in uniform "were the appropriate way to speak to that age group. But as time wore on, I started thinking: should we be speaking to this age group at all?"

Buck says that once he had signed up, recruitment officers gave him business cards to hand out to his friends. He says he was offered up to £150 for each recruit he could help persuade into uniform, which would be paid once they finished their phase one training.

The report – which drew on 41 existing studies into mental health problems among armed services personnel, as well as interviews with veterans – also indicates that young recruits from disadvantaged backgrounds are at greatest risk of suffering mental health issues. It found that 8% of Iraq war veterans who signed up without qualifications such as GCSEs had PTSD after their deployment, compared with 4% in the armed forces as a whole.

Les Gibbons, 55, joined the Royal Marines when he was 16 and remembers seeing the army as the "only way out" of the deprived community where he grew up in Petersfield, Hampshire. "I think the military cashed in on the lack of opportunities."

He left the forces at 22, as soon as he was legally able to, and has since become a social worker. "I see lots of kids with mental health problems who want to join the army – this makes me very uncomfortable," he said.

David Gee, the report's author, said: "When it comes to the trauma of warfare, recruits from the poorest backgrounds face a perfect storm of pre-existing vulnerability and greater battlefield exposure. Recruiting 16-year-olds into the infantry puts the most vulnerable group in roles most exposed to trauma when they turn 18 and are sent to war."

An MoD spokesperson said: "It is nonsense to claim that the armed forces recruiting specialists receive any form of commission for each recruit. All armed forces marketing material contains realistic firsthand accounts of experiences of operations, which is precisely what our potential recruits want to see and hear.

"During the recruitment process our potential recruits are well informed and constantly reminded about the risks linked to serving in the armed forces.

"This report completely ignores the benefits and opportunities that a military career offers young people. It provides them challenging and constructive education, training and employment, equipping them with valuable and transferable skills for life.

"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."

Haymarket, which Pendleton worked for, said that it has always worked to the MoD's brief and approval on recruitment materials.