During the long-term service period of the municipal solid waste (MSW) landfill, the strength parameters of the waste body change as time elapses. In analyzing the stability of the MSW fill, we decompose the waste body into layers according to the age, and propose two failure mechanisms based on the upper bound theorem of limit analysis, including the rotational and translational mechanisms. To confirm the above two mechanisms, a comparative analysis is performed on a slope composed of horizontally soil layers, and then the case of the landfill slope layered by aging is analyzed by assuming the two mechanisms above. It is shown that the factor of safety and failure mechanism by the upper-limit solution of limit analysis are very close to those deduced from the reduction finite element analysis, and the critical failure mode obtained by the multi-block mechanism is more reasonable. In addition, the stability of the Coll Cardús MSW landfill in Spain with complex layers by aging is analyzed, and the results are consistent with those of the reduction finite element analysis. Our results indicate that, as the fill age increases, the landfill slope will become more and more stable in the absent of pore pressures. The fill age can be divided into low, middle and high levels. The safety factor of the landfill slope layered by aging is between those of the homogeneous slopes with middle to high aging levels.