Polycystic ovarian syndrome (PCOS) is known as one of the most frequent endocrine diseases in women worldwide. However, this term does not completely capture the diversity of clinical signs associated with this syndrome e.g., menstrual irregularity and clinical features of androgen excess, which are though commonplace in women with PCOS, they are not included under the definition of PCOS, limited to polycystic ovarian morphology (PCOM). Utilizing the most globally accepted criterion used today in the diagnosis of PCOS, the authors of this article review and discuss the historical and current context of evidence as well as their limitations. This review addresses the phenotypic approach and age-dependent aspects of PCOS in adolescents, adult and peri/postmenopausal women, as presented in the NIH (1990, 2012), Rotterdam (2003), AE-PCOS Society (2006) consensuses and in the latest evidence-based international guideline (2018). Global data on the epidemiology of PCOS, including prevalence and distribution of polycystic ovarian syndrome phenotypes, is also analyzed in the article. Lastly, the authors discuss the importance and current need to perform more epidemiological studies focused on PCOS.