The influence of hydrogen on the long-time aging response of a commercial Al-Mg-Si alloy was investigated using transmission electron microscopy (TEM) and atom probe tomography (APT). The precipitate volume fraction and composition remain unchanged under H2 atmosphere but the coarsening kinetic is significantly reduced leading to a higher precipitate density and a higher micro-hardness. Classical Oswald ripening theories combined with a model accounting for strengthening contributions indicate that the underlying mechanism is controlled by an increase of the vacancy migration energy by hydrogen. A similar phenomenon was also observed during natural ageing at room temperature where the hardening response is delayed under hydrogen environment. To clarify atomic scale mechanisms, the influence of hydrogen on the formation and growth of GP zones in a model Al-Cu alloy was investigated by high resolution TEM and using ab initio calculations. Incorporation of hydrogen in the alloy leads to a slower the hardening kinetic during natural aging and experimental data clearly show that it is the result of a slower growth of GP zones. According to ab initio calculations, hydrogen atoms attracted by vacancies significantly reduce the mobility of copper atoms. Two mechanisms operate depending on the relative position of H and Cu near the vacancy. When H is in the path of the atom that exchange with the vacancy it increases the energy barrier and when H is dissociated from the vacancy it creates less stable state.