Age-hardenable Al-Mg-Si alloys containing additional elements of Ag, Cu, Pt or Pd were processed by high-pressure torsion (H PT) and they were subsequently aged at a temperature of 373 K for up to a total period of 5,400 ks. It was found that, in all alloys, the grain sizes after HPT were refined to 300-430 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent aging treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying elements. In the Cu-added excess Mg alloy, the hardness increase was large and the higher hardness retained for longer aging time when compared with those of non-added alloys. It was suggested that the precipitation of a few particles within a single grain with the size of a few hundred nanometer contributes to appreciable age hardening.