Some teenagers have suffered from long Covid for more than two years, researchers have found.
However, the majority who had long Covid three months after first testing positive will recover, according to the largest study of its kind.
Experts said that more work is needed to understand why some children still have ongoing health problems two years after infection.
Researchers from across the UK, led by experts from University College London (UCL), examined data on thousands of older children and teenagers.
Young people aged 11 to 17 were asked about their health three, six, 12 and 24 months after taking a PCR test for the Covid virus between September 2020 and March 2021.
Out of 12,632 teenagers and older children who participated in the study, some 943 tested positive and provided answers at three, six, 12 and 24 months after their original test.
Of these, 233 were deemed to have long Covid three months after initially testing positive.
At six months, 135 continued to meet the research definition of long Covid, according to the paper which has been published in the journal Nature Communications.
The children and teenagers were defined as having long Covid if they had more than one symptom - for at least three months - of tiredness, trouble sleeping, shortness of breath or headaches, alongside problems with either mobility, self-care, doing usual activities, having pain or discomfort, or feeling very worried or sad.
After a year, 94 were still deemed to have long Covid.
This reduced to 68 two years after the initial positive test, according to the research which is the world's largest "longitudinal cohort study" on long Covid in children.
Experts highlighted this means that 70% who had long Covid three months after the infection had recovered by two years. But 30% had not.
These children reported an average of five to six symptoms every time they reported on their health.
The most common symptoms were tiredness, trouble sleeping, shortness of breath and headaches.
"Our findings show that for teenagers who fulfilled our research definition of long Covid three months after a positive test for the Covid virus, the majority have recovered after two years," said study chief investigator and first author, Professor Sir Terence Stephenson, from the UCL Great Ormond Street Institute of Child Health.
"This is good news but we intend to do further research to try to better understand why 68 teenagers had not recovered."
Older teenagers and the most deprived were more likely to have long Covid, experts found.
And girls were more likely as boys to have long Covid, though the study did not account for menstruation and pre-menstrual syndrome.
The authors were also keen to stress that the children tested positive before the Delta and Omicron variants of the virus became dominant, so the findings may not reflect the long-term effects of these variants.
Commenting on the findings, Dr Nathan Cheetham from King's College London said: "This study again shows that health conditions like long Covid tend to affect the most disadvantaged in society, both young and old.
"These results reinforce the need to address the underlying causes of ill health, such as poor housing conditions, financial stress and unequal access to health care services, if we want to shift focus towards preventing illness before inequalities such as those found in this study arise."
The study was funded National Institute for Health and Care Research and UK Research and Innovation.
