Alien invasive woody plant species, particularly those associated with nitrogen-fixing bacteria, are of special concern, because they may cause drastic changes in entire ecosystems. Although most of invasive species of Lithuania originate from other continents, one of these, Cytisus scoparius, is native to Europe. Fast spread of this species in Lithuania and its invasion to forest, coastal and continental sand, occasionally grassland habitats, stimulated us to study demographic structure of its populations. The aim of this research was to evaluate density, life stage and age structure of the populations, and to analyse relationships of the age of individuals with their life stages, height and stem diameter. Five populations were studied in the western, southern and eastern parts of Lithuania in July–August 2017. For the study, we selected stands of this species occupying at least 0.1 ha in a uniform habitat. We applied sampling plot method, and studied at least 100 individuals in each population, excluding seedlings. The age, life stage, height and stem diameter of 583 individuals were studied. The average density of individuals ranged between 5.15 individuals/m2 and 15.40 individuals/m2. Juvenile individuals reach the stage of vegetative adults on the second or third growth season, occasionally on the third year they reach even the stage of generative adults. The average age of juveniles was 1.10 years, of vegetative adults it was 2.99 years and 6.26 years of generative adults. The oldest generative individual was 28 years old. Linear regression analysis of the relationship between the plant age and the stem diameter revealed that the age predetermined the stem diameter variance by 81.0% in the studied populations. This study revealed that densities of C. scoparius individuals of all life stages in disturbed and undisturbed habitats were sufficient to sustain longevity and further expansion of populations. Therefore, appropriate measures for control and eradication of this species should be applied, particularly in the habitats of high conservation value.