Purpose: To assess the prevalence of periodontitis in young populations (previously termed aggressive periodontitis - AgP) and report on the earliest known occurrence of this disease. Materials and Methods: A search was performed covering the last 18 years utilising the following databases: Medline (Ovid), PubMed and Embase. Four reviewers evaluated each study. Review findings were summarised using the PRISMA Statement for reporting and Joanna Briggs Institute (JBI) Critical Appraisal tools for quality assessment, respectively. Twenty-two articles were included in this systematic review, consisting of 6 prevalence studies and 16 case reports. Only prevalence studies were considered for prevalence estimates. Results: The average reported prevalence of periodontitis in young populations was 1.7% (ranging from 0.66% in Argentina to 5.9% in Israel). The prevalence was higher for the localised form of this disease. Permanent teeth were the most common dentition affected (114 out of 115 affected patients). In terms of age, the prevalence was 0.6%, 0.8% and 1.6% for the age groups 2-12, 20-25 and 13-20, respectively. There was no significant difference noted between males and females. The earliest age diagnosed with periodontitis was 3 years 7 months. Conclusion: The prevalence of periodontitis in young patients is ranges widely, which could indicate populational predispositions, underdiagnosing or lack of standardisation in diagnosis. Keywords: aggressive periodontitis, demographics, epidemiology, misdiagnosis