The UK is the only country in Europe and Nato that still recruits 16 year old children into its armed forces. Kids can even start the application process at just 15 year and 7 months. Ad campaigns like this should be seen in that context.
Former head of Army recruitment strategy Colonel David Allfrey said, “[It] takes a 10-year span. It starts with a seven-year-old boy seeing a parachutist at an air-show and thinking, “That looks great!” From then the army is trying to build interest by drip, drip, drip.”
The poorest UK regions supply large numbers of these child recruits. The army says it looks to the youngest recruits to make up shortfalls in the infantry, by far the most dangerous part of the military. (Infantry fatality rate in Afghanistan was seven times higher than rest of the military.)
A study (linked here) by human rights groups Forces Watch and Child Soldiers International in 2013 found that soldiers who enlisted at 16 and completed training were twice as likely to die in Afghanistan as those who enlisted aged 18 or above. That's because the youngest recruits are often enlisted into front-line combat roles like infantry, tanks or artillery. In fact, the very youngest recruits - aged between 16 and 16 years, 3 months, are only allowed to join combat roles.
Those child recruits are not supposed to be sent to war until they reach 18 (often on the day they turn 18). But that doesn't always happen, and British children were mistakenly sent to war in Iraq and Afghanistan on multiple occasions.
The Ministry of Defence has stated that its aim in getting children to join the military at 16+ is to recruit young people "before they have made other lifestyle choices". Young military recruits are less likely to be aware of the mental and physical health risks of their prospective career, unlikely to be told of them, and unlikely to be able to seriously consider the real-life implications at that age.
Among veterans who left the forces in the last ten years, levels of PTSD, alcohol misuse, common mental disorders, self-harm, and suicide are substantially higher than they are among civilians. Risk varies widely w/ socio-economic background & is greatest for young people from poor backgrounds, while those enlisting at 16 and 17 are most likely to be worst affected. (fig 4, p25 Study link)
Suicide rate for 16-20 year old males in the armed forces has been 82% higher than for civilians at same age.
Source: https://www.theguardian.com/uk-news/video/2013/oct/28/british-army-young-recruit-video