And the delay in treating them ends up costing society more than £105billion a year, the Centre for Mental Health (CMH) has found.
The research found that ignorance of mental health issues and the lack of support for children and parents are the main causes of the decade of delay. In that time, the illness can worsen and escalate into crisis, according to the centre.
The CMH used international studies to show the average age a mental health difficulty begins is 10 years prior to the point people begin receiving treatment.
Andy Bell, the centre's deputy chief executive, says figures around prevalence prove that problems among children are more common than most realise.
"They show that three children in every classroom are affected at any one time," he says.
"Yet only a minority of children with mental health problems get any help."
One in ten children are affected with mental health problems, the centre's analysis shows. It means there are an estimated 900,000 children between the ages of five to 16 who are suffering and most will do so for years before getting any support at all.
Rachel*, now 18, is just one of the tens of thousands of young people who struggled to find help. "I just wanted someone to care and believe," she told The Huffington Post UK.
"To say 'you're not worthless, not ugly, incapable. You shouldn't be treated like this'. If someone just told me this, it would have made so much of a difference."
She was six when she first showed symptoms of what she now knows was mental illness.
Rachel started hearing voices in her head in her early teens and at 14 was diagnosed with depressive disorder, post traumatic stress disorder and emerging borderline personality disorder.
Her experience - trips to A&E when she felt suicidal and having to travel far for treatment because there were no hospital beds in her area - has made her a passionate advocate of earlier intervention, which she thinks could have prevented what happened to her.
Now training to be a mental health nurse, she says: "If I can help one person to realise that there is nothing wrong with them, then it'll be worth it."
Rachel's suffering is borne out in the centre's research, which shows about 70,000 11-year-olds have a mental health condition. Twice as many will have had a problem at some point.
But an independent taskforce this week reported the NHS spent £9.2bn on mental health in England in 2013/14, a small fraction of the total spending.
In fact, separate estimates from the charity Young Minds, claim spending on children's mental health is around 7% of the mental health budget and just 0.7% of the total NHS budget.
The Mental Health Taskforce called for an extra one million people to be receiving treatment for their mental health conditions by 2020. In response, the NHS pledged to commit another £1bn in funding by then, acknowledging that mental health has been "the NHS' poor relation".
Mental Health Taskforce:
- The NHS spent £9.2bn on mental health in England in 2013/14, a small fraction of the total spending
- Children from low income families were at greater risk of developing a diagnosable mental health problem, three times more so than children from the richest families
- Children who develop "conduct disorder" - persistent, disobedient, disruptive and aggressive behaviour - were twice as likely to leave school without any qualifications, three times more likely to become teenage parents, four times more likely to develop a drug problem and 20 times more likely to go to jail
The Centre for Mental Health says treating mental illness through health and social care combined costs the country £21.3billion but billions more is lost in economic output and a further £53.6billion is lost in "human cost", the value put on damage to sufferers' quality of life.
In all, the centre puts the annual cost to society at a conservative £105billion - almost as much as we spend on the entire NHS in England and the equivalent of about £1,500 for every man, woman and child in Britain.
Yet, experts insist, we could slash that bill if we were prepared to put more resources into helping heal young minds.
The centre's associate director for children and young people Lorraine Khan said: "Good mental health is shaped very early by the first spark of life. Childhood experiences and exposure to risks for poor mental health make some children especially vulnerable to both emotional behavioural problems.
"The longer they are exposed to risks such as neglect, abuse, bullying and the effects of poverty, the more their life changes are undermined."
The Case For Intervention
Anti-bullying initiatives in schools could cost just £75 a pupil, yet can save £14 for every £1 spent, a separate report for the centre estimates.
Functional Family Therapy, through which families receive 30 hours of therapy over three months, can save £12 for every £1 it costs.
Family Nurse Partnerships, where nurses carry out home visits for a child from birth to the age of two, can combat behavioural problems and save £2 for every £1 spent.
Prof Peter Fonagy, chief executive of The Anna Freud Centre, a leading child mental health research and treatment centre, said intervention was most effective among primary school age children.
"As kids get older, the effectiveness of intervention is more limited," he says. "Interventions in primary schools could create the environment for children and young people that make the emergence of problems, such as anti-social problems, conduct disorder and depression, less likely.
"That's where you are likely to achieve a considerable amount of benefit."
Three-quarters of adult mental illness, excluding dementia, begins before the age of 18, official estimates show. Half begins before the age of 15.
But three-quarters of mental health conditions in young people are undiagnosed, according to a study published last year.
Despite the advantages of early intervention, budget cuts have made it less likely, according to Bell.
"The amount of intervention now is very low. With three-quarters of need unmet, you can almost only get better. But what we're seeing in lots of other places is, it's getting worse," he says.
"Children's mental health has been starved of resources to a degree that adult mental health hasn't and adult mental health isn't exactly swimming around in extra money."
Furthermore, he said, there was talk of "wide-scale disinvestment" in measures such as mental health support in schools.
"What that means," says Bell, "is we're shifting towards later intervention, crisis help and children escalating into pretty difficult circumstances that earlier intervention would've prevented."
He said a much-touted extra £1.25billion for children's mental health, to be spent over five years, should be invested in "a more thorough form of early intervention, from perinatal mental health to assessing children's needs in primary school, providing more credible mental health support to teenagers".
He added: "We can get better experiences of care and better care, which over the life course would be of much, much less cost."
Prof Fonagy warned that budget cuts had shifted the focus to children with the most acute needs, meaning some victims of serious mistreatment would be denied help.
More than a fifth of children referred to NHS specialist mental health services are rejected for treatment, according to the NSPCC.
Some of these tens of thousands of children have suffered abuse and, the charity says, failure to tackle their problems means the damage can last a lifetime.
Many of them miss out only because the problems they have only become apparent later in life.
Prof Fonagy said: "Services have experienced a year on year about 10% increase of demand against a background of more and more limited funding. That will focus on the children most in need."
The extra £1.25billion was the first injection of new money for children's mental health since 2010, said Prof Fonagy, adding that spending on it had fallen in real terms since then.
He added it would not substantially boost the proportion of the mental health budget that goes to children and said it would remain "insufficient for the level of need, particularly if you regard early intervention as a priority".
*Names have been changed to protect identities
If you or someone you know have questions about any of the issues raised in this article, get in touch with the Rethink Mental Illness Advice and Information Service
Young Minds Matter is a new series designed to lead the conversation with children about mental and emotional health, so youngsters feel loved, valued and understood. Launched with Her Royal Highness, The Duchess of Cambridge, as guest editor, we will discuss problems, causes and most importantly solutions to the stigma surrounding the UK’s mental health crisis among children. To blog on the site as part of Young Minds Matter email email@example.com.