Their survival rates for cancers such as leukaemia are much lower than in younger children, says a report in the Lancet Oncology.
The researchers suggest differences in tumours, delays in diagnosis and treatment and a lack of clinical trials for that age group are to blame.
Cancer Research UK said it was crucial to find out what was going wrong.
The study analysed data from 27 countries on nearly 57,000 childhood cancers and 312,000 cancers in teenagers and young adults.
Overall, five-year survival rates were higher in teenagers and young adults at 82% compared with 79% in children.
But those better prospects were largely driven by the older age-group getting cancers with a better prognosis.
The overall rate concealed areas of concern where survival was "significantly worse" for eight cancers commonly found in both age groups.
The five-year survival rates for:
Dr Annalisa Trama, from The National Institute of Cancer in Milan, Italy said: "The good news is that the number of children, adolescents and young adults surviving for at least five years after diagnosis has risen steadily over time in Europe.
"However, we found that adolescents and young adults still tend to die earlier than children for several cancers common to these age groups, particularly blood cancers."
Dr Alan Worsley, from Cancer Research UK, said: "While it's great news that the number of children, teenagers and young adults surviving cancer continues to improve, it's also clear that for some cancers, survival in different age groups is improving faster than in others.
"We need to find out whether adolescents are faring worse because of how their cancer is managed in the clinic or whether it's because the underlying biology is fundamentally different at these ages.
"Answering these questions is a big part of the reason why we've launched the Cancer Research UK Kids and Teens campaign."
Too many teenagers and young adults are dying of some types of cancer, a Europe-wide report warns.