A second study looked at 95 patients with a variety of tumours and found the interval ranged from two to 192 weeks, with an average wait of 9.5 weeks until diagnosis.
A third study by Sam Smith, a teenage cancer nurse at Manchester's Christie Hospital, surveyed 200 young people, and found 80% sought help within four weeks of noticing pain, a lump or swelling, weight loss or tiredness.
Almost all of them had two or three of these symptoms.
But 54% of patients with Hodgkin lymphoma, 59% with brain tumours and 46% with bone tumours visited their GP four or more times with symptoms before referral.
Professor Eden said: "It would appear that when we compare these data with studies of children with cancer, teenagers and young adults do face greater delays in diagnosis, particularly for bone and brain tumours and Hodgkin lymphoma.
Teenagers and young adults do face greater delays in diagnosis
Professor Tim Eden, University of Manchester
"And the older the patient, the longer the delay.
"In our studies the professional interval has always been longer than patient symptom interval."
He said there were delays at both GP and hospital levels.
Professor Eden said it was important to raise awareness amongst doctors of the signs of cancer in teenagers and to speed up their response.
"Whether this will improve survival remains unclear but it will reduce anxiety, anger and distrust of doctors."
Professor Steve Field, chairman of the Royal College of GPs, said: "It's very easy to look back and say 'a GP should have seen something' but cancer in this age group is extremely rare."
But he added: "When a patient is found to have cancer, the GP should look back and see whether or not there were signs that could have been picked up earlier."
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