Telomere length (TL) is a DNA marker of biological age in humans. Shorter TL (signifying older biological age) associates with higher risk of age-related disease such as coronary artery disease. Whilst TL is strongly heritable, it is also associated with lifestyle and environmental factors, such as diet, exercise and smoking in adults. However, recent studies have proposed that TL is "set" in early life and that environmental/lifestyle exposures may have more effect on TL during childhood. For example, adult smokers have, on average, shorter TL than non-smokers but smoking does not increase TL loss over time in adulthood. Therefore, the relationship between smoking and TL is more complex and is likely established at an early age. One possible explanation is that childhood exposure to smoke from parental smoking both shortens TL in the child and increases the likelihood of them smoking in later life. These same relationships may also be seen for other traits such as diet and exercise, traits that in adults are often influenced by childhood experience. Currently there are few studies that are able to address such questions. By measuring TL in children and young adults at large scale (minimum 2,900 per age group, 7, 17 and 24 years) we can more accurately explore the relationships between lifestyle and environment in childhood with TL. This will provide important information about how TL is influenced in early life and how early life exposures influence disease risk in adulthood through a biological ageing mechanism.