The objective of this study was to assess the degree of long-term waste maturation at a closed landfill (Etueffont, France) over a period of 21 years (1989-2010) through analysis of the physicochemical characteristics of leachates as well as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and metal content in waste. The results show that the leachates, generated in two different sections (older and newer) of the landfill, have low organic, mineral, and metallic loads, as the wastes were mainly of household origin from a rural area where sorting and composting were required. Based on pH and BOD/COD assessments, leachate monitoring in the landfill's newer section showed a rapid decrease in the pollution load over time and an early onset of methanogenic conditions. The closing of the older of the two sections contributed to a significant decline for the majority of parameters, attributable to degradation and leaching. A gradual decreasing trend was observed after waste placement had ceased in the older section, indicating that degradation continued and the waste mass had not yet fully stabilized. At the end of monitoring, leachates from the two landfill linings contained typical old leachates in the maturation period, with a pH ≥ 7 and a low BOD/COD ratio indicating a low level of waste biodegradability. Age actually contributes to a gradual removal of organic, inorganic, and metallic wastes, but it is not the only driving factor behind advanced degradation. The lack of compaction and cover immediately after deposit extended the aerobic degradation phase, significantly reducing the amount of organic matter. In addition, waste shredding improved water infiltration into the waste mass, hastening removal of polluting components through percolation.