This research investigates simultaneous strengthening by grain refinement and fine precipitation in age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd. The alloys were solution-treated and processed by high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa. They were aged at a temperature of 373 K for up to a total period of 6.7 hours. Vickers microhardness was measured after selected periods of aging and the microstructures were observed by transmission electron microscopy. It was found that, in all alloys, the grain sizes after HPT were refined to 300-400 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent ageing treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying compositions.