One in 13 young people in England and Wales experiences post-traumatic stress disorder by the age of 18, the first research of its kind suggests.
A study of more than 2,000 18-year-olds found nearly a third had experienced trauma in childhood.
And a quarter of these then developed PTSD, which can cause insomnia, flashbacks and feelings of isolation.
Researchers say, with many young people not receiving the support they need, the study should be a "wake-up call".
The study, published in The Lancet Psychiatry, found slightly more than half of those who had had PTSD - an anxiety disorder caused by very stressful, frightening or distressing events - had also experienced a major depressive episode and one in five had attempted suicide.
But only the same proportion - one in five - had been seen by a mental health professional in the past year.
Lead researcher Dr Stephanie Lewis, a Medical Research Council-funded researcher at the King's College London Institute of Psychiatry, Psychology and Neuroscience, said: "Providing effective treatments early on could prevent mental health problems continuing into adulthood."
Surgery as a baby 'led to PTSD'
Flo Sharman, 20, had a breakdown at the age of eight that left her housebound.
But it was only when she was 16 that neurologists concluded that breakdown was linked to something that had happened to her as a baby.
At four months old, Flo had a life-saving operation to remove parts of her stomach - to treat her serious acid reflux, which affected her ability to breathe.
The condition had led to her needing to be resuscitated on many occasions, Flo said.
Doctors now believe her breakdown was caused by PTSD, a delayed reaction to the trauma she experienced as a baby, even though she has no conscious memory of what happened.
She says: "When I had that life-saving surgery and the traumatic experiences in hospital, my parents weren't told you could have post-traumatic stress disorder because of the surgery and the time in hospital.
"They had no idea that this mental breakdown would happen. It was a real shock."
Flo still has PTSD, which she says many people wrongly think affects only those in the armed forces.
"People don't really associate PTSD with a young child - and that has to change," she adds.
'Falling through the gaps'
Participants in the study were judged to have had PTSD only if they had had all of the following symptoms for at least a month:
- reliving traumatic events through distressing memories or nightmares
- avoiding anything reminding them of their trauma
- feelings of guilt, isolation or detachment
- irritability, impulsivity or difficulty concentrating
Experiences of childhood trauma included assault, sexual assault, injury or an event that had affected someone they knew but they had not directly witnessed.
Senior researcher Prof Andrea Danese, from the King's College London Institute of Psychiatry, Psychology and Neuroscience, said: "Our findings should serve as a wake-up call.
"Childhood trauma is a public-health concern - yet trauma-related disorders often go unnoticed.
"Young people with PTSD are falling through the gaps in care and there is a pressing need for better access to mental health services."
What is PTSD?
- Being caught up in a traumatic event that is overwhelming, frightening and life-threatening can lead to PTSD
- The symptoms usually start within a few weeks of the trauma but they can start later
- After the traumatic event, people can feel grief-stricken, depressed, anxious, guilty and angry
- People may have flashbacks and nightmares
- People may be 'on guard' - staying alert all the time
- Physical symptoms can be aches and pains, diarrhoea, irregular heartbeats, headaches, feelings of panic and fear, depression
- People may start drinking too much alcohol or using drugs (including painkillers)
Source: Royal College of Psychiatrists
Prof Danese said people should not be "alarmed" by the study's findings and it was normal to have some psychological symptoms after trauma.
In the "vast majority" of cases, these symptoms would recede in a matter of days or weeks, he said.
But if children and young people had them for over a month, parents should seek help from their GP.
PTSD can be successfully treated - even when it develops many years after a traumatic event - with treatments including talking therapies and antidepressants.
Dr Tim Dalgleish, from the University of Cambridge, who was not involved in the research, said the results of the "landmark study" were "sobering".
"Of particular concern is the relatively small proportion of affected youth who go on to access formal support or mental health services and the findings are a further wake-up call that service provision in the UK for children and adolescents dealing with the aftermath of trauma is woefully inadequate," he said.
A Department of Health and Social Care spokeswoman said: "The NHS Long Term Plan has committed to prioritising increasing the funding for children and young people's mental health services faster than all other funding.
"As a result, 345,000 more children and young people have access to mental health services and support in schools and colleges, young adults will receive better support until the age of 25 and crisis care will be provided through NHS 111, 24 hours a day, seven days a week."