At 16 you are not old enough to vote, buy a pint in a pub or ride a motorbike. Yet you can join the armed forces, and commit yourself for four years beyond your 18th birthday. On becoming legally adult you can then be sent to the frontline in Afghanistan. A 16-year-old soldier can train with live ammunition, yet when he goes back to barracks in the evening he isn't old enough to rent an X-rated DVD of Apocalypse Now, a film dealing with the horrors of war – because it is too violent.

Notions of childhood change. During the siege of Mafeking in 1900, Robert Baden Powell recruited 12-year-old boys to deliver messages under fire. They wore khaki and their leader was the 13-year-old Warner Goodyear. But today Britain is the only European country to recruit into the regular army at 16. Perversely those young recruits are required to serve two years longer than those recruited at 18. Far from being a curious legal relic, this rule was re-introduced by the Labour government in 2008. After a six-month "cooling-off" period there is no right to leave. While "unhappy minors" may leave at the discretion of their commanding officer, the fact that there is no "discharge as of right" leaves them uniquely open to bullying and that bullying is more serious if it happens because they cannot leave.

The situation of 16-year-old soldiers is sometimes compared to that of apprentices. Yet in what other 21st century apprenticeship can a breach of discipline lead to a court martial and time spent in military prison? In what other apprenticeship do you face such dangers? How many apprentice carpenters, brick layers or plumbers are found dead whether shot in the head or hanging from a beam? Yet that is what happened to four young recruits training at Deepcut barracks. A carpenter's skills are a guarantee of security in an economic downturn. There are more limited openings for trained marksmen. An infantryman returning from Helmland province has no guarantee of a job. David Cameron's call for a "national change" in attitude towards mental health problems among former soldiers is highly welcome, but could his proposed 24-hour helpline be extended to soldiers who are currently serving or training in barracks?

While the UK no longer has conscription, those joining the army at 16 often come from the poorest and least educated backgrounds. For youngsters without other jobs to go to, a career in the army may be hard to resist. What other choices do they have? It is true that 16- and 17-year-olds are no longer deployed to conflict zones but decisions made as a child have irrevocable consequences as an adult. At the moment a young person making a decision at 16, with his parents' consent, has no right at the age of 18 to review that decision with an informed conscience.

The army is in loco parentis to its under 18-year-olds. Under the UN Convention on the Rights of the Child, the army is obliged to consider the best interests of the child. Yet there is little independent oversight of this responsibility. For a young soldier to write to the children's rights commissioner would be bravery indeed. In enlisting in the army recruits become subject to military discipline under subsidiary legislation made under the Army Act 1955. But how many teenagers read subsidiary legislation?

The present situation is a throwback to the 19th century era of indentured labour. A consistent 21st-century understanding of the age of adult responsibility would mean that formal enlistment into the army takes place at 18 – the age of legal responsibility and not before. In the meantime, only a right of discharge for all those under 18 and a requirement that 18-year-olds make a clear and informed choice, on their 18th birthday, will conform to 21st century standards of human rights.