Substantial natural aging occurring on the time scale of less than 1 month was observed in solution-treated Mg–Gd and Mg–Tb alloys. Solution treatment at 500 and 530 °C for Mg–Gd and Mg–Tb alloys, respectively, leads to dissolution of Gd or Tb in the Mg matrix. By quenching of solution-treated alloys to room temperature a supersaturated solid solution of Gd or Tb in Mg was formed. During subsequent aging at ambient temperature the hardness of quenched alloys increases, most probably, due to agglomeration of dissolved Gd or Tb atoms into small clusters. Positron lifetime spectroscopy combined with coincident Doppler broadening revealed that quenched alloys contain vacancies bound to Gd or Tb atoms. Quenched-in vacancies facilitate clustering of Gd or Tb solutes and disappear from the samples when the clusters are fully developed.