More than 2,000 children born in England and Wales in 1994-1995 took part in the study, and were assessed with interviews.
The researchers concluded that one in four children who had experienced trauma had PTSD when they reported symptoms like reliving negative and scary experiences, distressing nightmares, avoidance of anything that reminded them of the trauma, isolation and detachment, irritability, impulsivity, guilt, and difficulty concentrating. That's nearly 8% of young people overall.
The results also showed that just one in five children had seen a mental health professional and one in three had talked to their GP about their symptoms in the past year.
"Our findings should serve as a wake-up call," said Andrea Danese, a senior researcher at the Institute of Psychiatry, Psychology & Neuroscience. "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," she said, so there's an urgent need for them to have better access to mental health services.
The results also showed how young people with PTSD were more likely to have other mental health disorders. For example, half had self-harmed, and one in five had attempted suicide. Half had also experienced social isolation or loneliness.
Last July, Childline a counseling service for young people said it had received nearly 5,000 calls about loneliness in a year. This was a 14% rise on the previous year.
"Young people who have been exposed to trauma often have complex problems, which become increasingly difficult to assess and treat," said Stephanie Lewis, a clinical research training fellow at the IoPPN, and lead author of the study. "Providing effective treatments early on could prevent mental health problems continuing into adulthood."
PTSD can be caused by a range of experiences, from directly experiencing assault, injury, or sexual violation, to watching it happen to someone else. Three-quarters of young people who experienced sexual assault developed PTSD.
The researchers said better identification of the symptoms of PTSD is vital if more young people are going to get the mental health help they need.
"We encourage parents and carers to seek support from health professionals if their children are exposed to trauma and are suffering from distressing psychological symptoms," said Lewis.