Both population density and sex ratio shape competition for mates, resources and mating costs. Thus they may critically affect the intensity of sexual selection in the populations. Susceptibility to inter- and intrasexual competition, which changes with age in a large number of species, may additionally influence population response to these demographic factors. In this study, we monitored 16 seminatural populations of common lizards (Lacerta vivipara) to determine whether the reproductive output varied with male and female densities as a function of the individual sex and age. Our results suggest that the intensity of sexual selection was weaker in male-biased populations, supporting new theoretical models. In populations with a male-biased sex ratio, reproductive success was more equally distributed between males and, unlike female-biased populations, the choosiest females (middle-aged) did not obtain sires of higher quality than low-performance females. Our results also suggest that age may influence the intensity of sexual conflict. Middle-aged females (the class with the best performance) produced offspring with a lower body condition in male-biased populations, suggesting that they may be the preferred target of male harassment. By contrast, a male-biased sex ratio appeared to be beneficial for low-quality females, allowing these females to obtain higher quality sires and to produce offspring with a better body condition. These age- and sex-dependent responses to population density and sex ratio have important implications for population ecology and sexual selection.