Children with mental health problems are facing a delay of a decade between displaying their first symptoms and actually getting help, a new report has revealed.
Opportunities to offer timely and effective support to children are "often" being missed, according to the Centre for Mental Health report.
In the 10 years that many UK children wait to get help, problems become "entrenched" and escalate until they reach "crisis", the document states.
It warned that the longer a child is left without any help "the more it is likely to cast a shadow over their entire life".
While mental health problems among youngsters are common, awareness is poor and many attempts by parents to get help for their children are unsuccessful, the report states.
Three quarters of parents of children with a mental health problem seek help, but only one quarter of children receive any support, it added.
Lorraine Khan, associate director for children and young people at Centre for Mental Health, said: "Most common childhood mental health problems can be treated effectively. Early help is vital to have the best chance of success.
"We need to take every opportunity to support families and schools to build firm foundations for children's mental health.
"We need to raise awareness of the first signs of poor mental health and reinforce the importance of getting early help. And we need to offer effective and young people friendly help for every child of any age at the first signs of difficulty.
"Waiting for a child's mental health to deteriorate until it hits crisis point causes untold distress and damage to their lives and carries a heavy social and economic cost. We have to take action now to offer high quality help quickly to children and young people everywhere."
Commenting on the report, Brian Dow, director of external affairs at the charity Rethink Mental Illness, said: "A decade is too long for anyone to wait for treatment, but we are letting down a whole generation if we continue to neglect children and young people's mental health, due to inadequate education, limited support and stigma.
"These findings have long reaching implications, we know that half of those who go on to have a lifetime of mental health issues first experience symptoms by the age of fourteen and problems can snowball over time. And yet a recent report from Centre Forum found that a quarter of young people are turned away from services at this crucial time.
"We need to see better support for parents and increased mental health awareness in schools; as well as improved access to services to put an end to this waiting game for treatment."
A Government spokesman said: "No-one should have to wait too long for mental health care, or be sent away when in need.
"That is why we have introduced the first mental health access and waiting time standards in NHS history and are putting a record £1.4 billion into transforming support for young people in every area of the country.
"This funding will improve care in the community and schools to reduce waiting times and make sure young people get support before they reach a crisis point."