Longitudinal studies are ideal for investigating age-related developmental change. When age is the time metric, different types of longitudinal designs can be distinguished according to the distribution of ages at recruitment. In a single cohort design all participants start out at the same age, whereas a study that recruits all available individuals with initial age in a specified range can be regarded as an unstructured multiple cohort longitudinal design. An accelerated longitudinal design is a more structured multiple cohort design that takes multiple single cohorts, each one starting at a different age.

Cohort effects arise in longitudinal studies when there is a systematic difference between people born at different times. We consider the possible impact of cohort effects on different types of longitudinal studies, and some methods for detecting and dealing with these effects.