MPs have become a widespread environmental contaminant that has a big impact on aquatic environments. This review examines the lifecycle and aging processes of MPs in marine and freshwater habitats. Key sources of MPs are explored, including land-based inputs from industrial effluents, wastewater treatment plants, and terrestrial plastic waste. The aging of MPs in aquatic environments is analyzed, focusing on processes such as photo-oxidation, mechanical stress, advanced oxidation, and biodegradation. These aging mechanisms alter MPs’ size, shape, crystallization, thermal endurance, and surface functioning are examples of their physical and chemical characteristics. Such transformations impact MP toxicity and interactions with other pollutants. While biodegradable plastics offer a potential solution, their environmental degradation remains limited under many conditions. Future research directions are proposed, including improved toxicity assessments, life cycle analyses across environmental compartments, and development of degradation acceleration/deceleration techniques. Overall, this review synthesizes current knowledge on MP aging in aquatic systems and highlights key areas for further investigation to address this critical environmental challenge.