Under-representation of teenagers and young adults in clinical trials for cancer is acknowledged internationally and might account for the lower survival gains noted for this group. Little research has focused on strategies to increase participation of teenagers and young adults in clinical trials. We applied a conceptual framework for barriers to recruitment of under-represented populations to data for cancer clinical trials in teenagers and young adults. We did a systematic analysis of data for clinical trial enrolment in Great Britain over 6 years (2005–10), and reviewed the published work for the origins and scientific rationale of age eligibility criteria in clinical trials for cancer. Our Review revealed little scientific evidence for use of age eligibility criteria in cancer clinical trials. Participation in cancer trials fell as age increased. Between 2005 and 2010, participation rates increased for children and young people aged 0–24 years. The highest increase in participation was for teenagers aged 15–19 years, with smaller improvements in rates for 20–24 year olds. Improvements were related to five key criteria, the five As: available, accessible, aware, appropriate, and acceptable. In studies for which age eligibility criteria were appropriate for inclusion of teenagers or young adults or amended during the study period, participation rates for 15–19 year olds were similar to those for 10–14 year olds. We propose a conceptual model for a strategic approach to improve recruitment of teenagers and younger adults to clinical trials for cancer, with use of the five As, which is applicable worldwide for investigators, regulatory authorities, representatives in industry, policy makers, funders, and health-care professionals.