The idea of the Metaverse as the next iteration of the internet gets increasing attention. As the development is still in its infancy, maturity assessments of the Metaverse in general and its constituting virtual worlds could provide critical input to guiding research and development as well as investments. Based on a scientific definition of the Metaverse, eight core attributes for its virtual worlds are extracted. For each of these attributes, five maturity levels are defined. Thus, a Metaverse maturity model with eight attributes and five maturity levels is proposed. This model is then applied to assess the maturity of the virtual world Decentraland. This pilot assessment reveals that attributes mainly depending on strategic decisions reach higher maturity levels than those mainly depending on technological aspects. This indicates a solid strategic dedication of Decentraland to the Metaverse vision.