The impact of aging on the mechanical properties and microstructure of Al-6063 alloys were studied, using scanning and transmission electron microscopy, as well as tensile measurements. It was found that the 0.5% yield stress and the ultimate tensile strength increase at the beginning of the aging process, reach a maximum, and than decrease with increasing the aging duration. On the other hand, the uniform elongation, the total elongation, and the strain hardening factor decrease with increasing aging duration well after the material reaches the maximum strength (T6 conditions), reaches a minimum and than increase again. The final fracture area reduction also decreases to a minimum, which occurs simultaneously with the maximum of the ultimate tensile strength; then it increases with aging time. The final fracture area reduction is accompanied with morphology transition from transgranular shear rupture to a combination of transgranular shear rupture and intergranular dimpled structure. The intergranular rupture area increases with aging up to the minimum in the total elongation, and then decreases with aging duration. Aging is accompanied with the appearance of needle-like Mg2Si precipitates except in Precipitation Free Zones (PFZ) that are adjacent to the grain boundaries. The PFZ size depends on the annealing temperature, while the morphology and density of the precipitates depend on the annealing duration. A correlation has been established between the PFZ and the measured mechanical properties and fracture morphology. The impact of the Mg/Si ratio on the mechanical properties is discussed.