After six months of misdiagnosis, Stephen was eventually told he had bowel cancer. He was 15. A straight-A student, he had always wanted to be a doctor. Now 18, he says: "I've lost faith in the healthcare system." He visited his GP numerous times with "crippling symptoms" and went to A&E on five occasions. "I have a strong family history of bowel cancer. At one point, we asked the doctor if I might have cancer and demanded further scans. He said: 'No, definitely not, you're too young.'"

Over the past three decades, cancer survival rates have increased dramatically. But for teenagers and those in their 20s, the outlook is less promising – several reports have shown that improvements in outcomes for them do not match those among children and older adults.

To investigate this, the National Institute for Health Research has launched Brightlight, a cohort study that hopes to become the largest ever of young cancer patients (aged 13-24). It intends to recruit more than 2,000 people by next April. In earlier studies, two of its leaders, Professor Jeremy Whelan, a specialist in teenage oncology at University College London Hospital, and Dr Lorna Fern of the National Cancer Research Institute, found that one of the critical factors for the "survival deficit" is delayed or mis-diagnosis. Invariably, this leads to a poorer prognosis as the more advanced the cancer is, the harder it is to treat.

According to a study published in the BMJ in October this year, those aged 16 to 25 are twice as likely as older adults to have three or more GP consultations before being referred to a specialist and a 2010 survey found more than a quarter of young cancer patients had visited their GP at least four times, many presenting with multiple "alert" symptoms (lumps, swellings or persistent unexplained pain) before their eventual referral. More than a third of participants at this year's Teenage Cancer Trust (TCT) conference (the aptly named Find Your Sense of Tumour) were only diagnosed on emergency. Among many young patients there is overwhelming resentment of GPs; a sense that they were not taken seriously, their symptoms dismissed as adolescent fatigue, stress, or persistent hangovers.

It is true that cancer is rare among young people. But it is also the leading medical cause of non-accidental death among them – and the TCT says that in the past 30 years, cancer among teens and young adults has risen by 50%; for the first time, teenage cancer cases outnumber those of children. Young people are also known to contract the most aggressive forms, exacerbated by their growth spurts. Failure to detect the warning signs at an early stage can have devastating consequences, as Stephen's case shows; his cancer was recently diagnosed as incurable. Raising awareness, he says, is essential to dispel the myth that young people are immune – a situation not helped by NHS campaigns such as Be Clear on Cancer, which feature only older faces.

Greater awareness is also needed to enable GPs to spot the "alarm-bell" symptoms. Since younger patients tend to develop the rarer forms of cancer – leukaemias, lymphomas, sarcomas, germ-cell tumours and cancers of the central nervous system – there is an urgent need for more research to group the specific symptoms of these malignancies.

At the moment, teens and young adults are also seriously under-represented in clinical trials. In a 2008 study, Whelan and Fern found almost half of patients aged 10-14 participated in a trial, but this fell to 25.2% for 15 to 19-year-olds, and just 13.1% for those aged 20-24. For too long, they argue, clinical and research communities have failed to recognise teens and young adults as a distinct category with unique biologies, and they have been left "forgotten in the middle".

It is the same story when it comes to hospital care; teens and young adults have been identified as a "lost tribe", caught between paediatric services and those designed for older patients. The National Cancer Reform Strategy (2007) estimated that 70% were not treated in age-appropriate settings; many were left on children's wards, with the disturbance of crying babies, or isolated in bays with elderly patients and surrounded by constant bereavement.

Those lucky enough to be treated alongside their peers at a young-person unit describe the experience as a godsend. "I was a total mess before I found Teenage Cancer Trust," says Jasmine Singh, 22, who is recovering from Hodgkin's lymphoma. She was transferred to a TCT unit with specialist young-person cancer nurses and counsellors, as well as a team of support workers offering education and career advice – a lifeline for young sufferers who fall behind with their studies or training.

In 2005, government directives recommended that all 13 to 24-year-old cancer patients be treated in such units – but at the moment there are only 27 in the UK, and only around half of all young patients are treated in one. The same recommendations stressed the importance of "age-appropriate care", but there is no real consensus over what this means. The Brightlight study sets out to provide some answers. By interviewing young adults over a five-year period, it aims to gain a clearer insight into the type of care that benefits them the most.

One of the study's pioneers was Stephen himself, in his capacity as a young adviser at the National Cancer Research Institute. "I don't know how long I have left because I haven't asked," he says. He refuses to waste a moment on self-pity – in between chemotherapy treatments, he is too busy fundraising for TCT, determined to improve the outlook for other young people with cancer.

• For more information on brightlight, email brightlight@uclh.nhs.uk