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All organisms have specific habitat requirements that allow them to properly function in their environment. For many organisms, individuals shift habitat choice as they age because optimal habitats vary across life stages. Despite age-specific habitat use in a variety of taxa, identification of the causal factors driving such variation is limited by a lack of experimental studies. Field observations of the brown anole lizard, Anolis sagrei , show that juveniles use low, narrow perches whereas adults use relatively higher and thicker perches. We hypothesized that this variation is driven by interactions between age classes, rather than age-specific preference for microhabitat. We manipulated adult and juvenile densities in field enclosures with artificial trees to examine how inter-age class competition influences microhabitat choice. We predicted that juveniles would move to less desired microhabitats as adult density increased (i.e. individual behavioural response) and/or adults would negatively affect juvenile survival (via competition or cannibalism) in ways that would contribute to age-specific habitat use (i.e. natural selection). We found that adult males, but not females, reduced juvenile survival. However, neither adult male nor female density influenced juvenile microhabitat choice (i.e. perch height, width or substrate) via individual behavioural response or natural selection. We also tested whether juveniles influence adult microhabitat choice. As predicted, adults did not vary in microhabitat choice in response to juvenile presence. Our study provides a rare and robust assessment of the role of age- and sex-specific density in generating variation in behaviour and survival under natural conditions. [ABSTRACT FROM AUTHOR]