The first official figures since 2004 showing how common mental health problems are among children and young people were published on Thursday.

The statistics have been collated by NatCen Social Research, the Office for National Statistics and Youthinmind, and funded by the Department of Health and Social Care. They are likely to be seen as the definitive data on how many young people are struggling mentally, for example with anxiety, depression or hyperactivity.

The 2004 data captured the prevalence of diagnosed mental health conditions in five- to 15-year-olds in Britain, based on a survey of 7,977 of them. One in 10 of that age group had such a disorder, the researchers concluded. Children from lone parent or reconstituted families, or whose parents were unemployed or had no educational qualifications, were more likely to be suffering.

However, this time they have collected information on 9,117 children and young people only in England. While they have looked again at five- to 15-year-olds, they have also examined for the first time the mental health of preschool children (two- to four-year-olds) and also 17- to 19-year-olds.

Why is young people’s mental health in the news so much?

It has become an issue of real concern, in the media and to both politicians and NHS leaders, over the last five years in particular. It has prompted numerous inquiries, reports, recommendations and pledges by politicians and NHS leaders to improve the situation.

The children’s commissioner for England, Anne Longfield, published a new analysis of the situation on Thursday, for example. She found that only a “small fraction” of those who needed help were getting it from the NHS.

Last year, for example, 37% of the more than 338,000 under-18s referred to NHS child and adolescent mental health services (CAMHS) in England were denied any help at all because they were not accepted into treatment or were discharged after being assessed. While 31% did begin treatment within a year, the other 32% were still waiting at the end of the year.

While there has been some progress in NHS mental health care for this age group “there is still a vast gap between what is provided for children suffering from mental health problems and what is needed to treat them. The current rate of progress is still not good enough for the majority of children who require help,” Longfield said.

Javed Khan, the chief executive of Barnardo’s, said: “We’re no longer headed for a crisis; we’re already there.” Dr Bernadka Dubicka, the Royal College of Psychiatrists’ spokeswoman on these issues, said: “Too many children are still falling through the gaps in mental health care.”

What is the government doing about all this?

It was a priority issue for David Cameron’s coalition government, which promised to give NHS child and adolescent mental health services in England £250m more a year every year between 2015 and 2020.

Theresa May has made it an even higher priority because she sees treatment of people with mental illness as one of the “burning injustices” of British society she wants to tackle as prime minister. She published a green paper on the subject a year ago and unveiled ambitious-sounding plans to expand NHS services for troubled youngsters and do much more to identify problems early and prevent them becoming long-term, as most mental health problems among adults began in childhood. Under her plans schools will be given a key role.

However, children’s charities, the National Audit Office and the Commons education and health and social care select committees have all said that her and the NHS’s pledges are nowhere near ambitious enough, given what all the evidence shows is a big gap between the need for care and the NHS’s ability to meet that need. For example, even under the NHS’s existing plans, the proportion of under-18s needing care who get it will only rise from 25% to 35% by 2021.

But further details of action, possibly including new waiting time targets to tackle delays in receiving treatment that can be as long as 18 months, will be included in the NHS long-term plan, which is due out at the start of December.

How many children and young people have a mental health problem?

It was 10% of five- to 15-year-olds in 2004. The comparable figures for 2017 released on Thursday show a slight rise to 11.2%. Attention or blame is likely to fall on the ubiquity in young people’s lives of social media and its damaging effects, for example through online bullying.

But the causes are much more numerous and more complex and include poverty, family dysfunction, being the victim of a sexual assault, exam stress and many other reasons.

The most recent figures show that a total of 389,727 under-18s in England are being treated every year for some sort of mental health condition and that about 50,000 people that age are being referred, mainly by their GP, for help every month. However, many do not seek help and some families, frustrated by sometimes long delays, end up paying for private counselling.

Who is more affected, boys or girls?

The 2004 survey found that boys were more likely to have a mental disorder than girls. However, the number of girls needing hospital care for self-harm via cutting or poisoning has risen about threefold in the last decade or so, while the increase was less dramatic among boys.

What about eating disorders?

Conditions such as anorexia and bulimia can be very hard to treat and require a long spell in hospital in order to save a sufferer’s life. Anorexia has the highest suicide rate of any mental health condition; about 20% of deaths due to the disease were the result of someone taking their own life.

An eating disorder is most likely to be diagnosed in people aged 15-19. It has been estimated that two girls out of every 1,000 will be diagnosed with an eating disorder. Eating disorders are the commonest new-onset mental health disorder in adolescent girls after depression.

Families and mental health charities complain that sufferers often find it hard to get treatment, especially a bed in a specialist treatment unit, where a patient can end up spending a year or more. “There is a gulf between the level of need and the numbers accessing specialist treatment,” said a spokesman for the charity B-eat.

The number of under-18s being admitted to hospital for treatment in England rose 113% between 2010-11 and 2016-17, compared with a 92% rise among all age groups.

Do NHS child and adolescent mental health services have enough beds to cope with demand?

Often not. Bed shortages are common. Many hundreds of children and young people are sent away from their home area simply in order to get a bed in which to be treated, often for many months. Some end up several hundred miles from home, while dozens of patients with eating disorders are sent from England to Scotland every year to be treated in private hospitals in Glasgow and Livingston.

However, NHS England is making good progress in delivering on its pledge of increasing the number of what are called tier 4 CAMHS beds by 150-180. Scores of extra beds have been opened since 2016.