Deborah Orr: We should all be shocked by these stories of teenagers shot and stabbed on our streets

When Boris Johnson tripped, literally, on to the London stage, and spoke of crime being the issue that worried Londoners most, he delivered a perfect illustration of how easy it is to be the new guy. In making street violence the centrepiece of his campaign, he had echoed the rhetoric of David Cameron, with his claim that Britain is "a broken society".

Neither Ken Livingstone, nor Labour more generally, can afford the luxury of such high-octane, emotive observations, however much they would like to indulge in them. Leaders cannot promote panic, and talk fear up. Those who aspire to leadership can be a little more cavalier.

Nevertheless, in playing down the teenage wave of violence that has disturbed Londoners so much, Mr Livingstone seemed out of touch, when really he was just maintaining the degree of perspective that everyone living in the capital must maintain, in order to be able to get on with their lives. Likewise, the Government has been forced for some time into playing down its own worries about the phenomenon of extreme young street crime, with some occasionally absurd consequences.

The Home Secretary, Jacqui Smith, was pilloried for suggesting that Hackney was not a place where women could walk in perfect relaxation at night. The party chairman, Harriet Harman, was likewise ridiculed for appearing with the police on the streets of Peckham, in a protective jacket.

Yet despite what the local MP, Diane Abbott, says, it is realistic to suggest that the streets of Hackney are to be avoided at night whenever possible if you are a woman on your own. As for Ms Harman's armoured vest, as she explained at the time, she was merely donning the equipment which the police issued to her. They wear such clothing as a matter of course, and for the good reason that there are plenty of dangerous weapons on the streets. Ms Harman's protective attire, or lack of it, makes no difference to this reality at all.

If you live in Hackney, or in Peckham, or in various other parts of London, or of other inner city areas, you know perfectly well that these are indeed robustly "edgy" parts of town. You'd have to be singularly unaware of your surrounding to fail to spot it. The local papers are full of reports of such incidents. Yellow boards asking for witnesses to come forward because of a "fatal incident" are a regular fact of life.

Familiar streets turn up on the telly, offering baleful views of residential areas where children have lost their lives. Few people don't know someone who knows someone who has been affected by gun or knife crime. Who could consider a city where a couple of teenagers are killed every month to be one that doesn't have intractable social problems?

Right now, in the Old Bailey, a couple in what seems like an endless stream of murder trials in which the defendant is accused of killing a teenager, are progressing. One of the victims, 18-year-old Nathan Foster, was until his death a typical example of a young man from a family that rose above the ingrained social difficulties his community lives among, as mostly, people do.

He was a talented horse rider, who himself taught other children to ride, at a stables in the unlikely location of Coldharbour Lane, in Brixton, more than once the scene of riots. He also worked with the local council, providing homework help to younger children. He was shot six times in the street in what seems to have been a case of mistaken identity, during the aftermath of a row over a gold chain. The 17-year-old defendant cannot be named for legal reasons, and witnesses are being screened, with their voices altered electronically, as they give evidence.

The murdered child in the other trial, again in what seems certainly to have been a case of mistaken identity, 15-year-old Michael Dosunmu, was shot as he lay asleep in his own bed, with a Mac 10 submachine-gun. The older brother of the boy admits that he was part of a robbery gang, but denies he is what the South London Press calls "a large-scale drug dealer with a long list of enemies who might want to kill him". The two men on trial, 19-year-old Mohammed Sannoh, and 22-year-old Abdi Omar Noor, have been linked to the shooting by firearm discharge evidence, while Noor's DNA was found on a bullet casing from the murder weapon. Both men have alibis.

Who could fail to be appalled by these trials, in which the dead teenagers where attacked with such lethal certainty, by mistake? Operation Trident, the long-running black-on-black murder campaign undertaken by the Metropolitan Police, has had success in stemming organised crime among older people, who might at least have been described as "professional". But it is the sheer youthful amateurism that typifies the more recent manifestation of armed street violence that has the power to provoke widespread fear.

The young people caught up in this hoodlum fantasy-come-reality talk of having to arm themselves not because they are aggressive, but because they are frightened. The impression is of youthful swagger, rooted in a sort of macho play, driven by a lack of mature impulse-control, but accessorised with real weapons, not toys. There is consensus about the toxic mix of difficulties that creates the climate – poor housing, few job opportunities that offer the gaudy rewards seen among the criminals, lack of male role models, inadequate educational opportunities, and a materialistic culture that glamorises violence.

There is consensus too about some of the steps that need to be taken to tackle such a culture. But often they simply sound like the solutions that have been bandied around for decades. "Juvenile delinquency" has been a theme in the public discourse for most of many people's lives now, with its talk of getting people off the streets, providing young people with things to do and places to meet, and healthy sporting activities. These are the very things that Mr Johnson is talking about now.

Yet in Stockwell, the young man who runs the local football team comes twice a year round to the doors of the locals he knows he can rely on to donate, just as he has done for years. In Southwark, a museum bequeathed to the children of the borough has been earmarked for closure, despite its prolific use by deprived families around the famously troubled Old Kent Road. In Camberwell a popular adventure playground, shut down for health and safety reasons, may never reopen, if a local campaign to refurbish it fails.

While supporters of Mr Livingstone were at pains to point out how tiny the sums that went adrift in the funding of black community projects were, the fact remains that tiny sums are all that many worthy projects are looking for, in order to stay afloat. Mr Johnson may have benefited from his ability to make speeches about youth crime, safe in the knowledge that Henley-on-Thames has not been tainted under his watch by such blights. But it will be interesting indeed to see how long he feels he can afford to make this hugely challenging problem his trump card.

d.orr@independent.co.uk