This study presents age-hardenable, fine-grained AlMgZnCu crossover alloys intended for superplastic and quick plastic forming processes. The study utilizes T-phase (Mg32(Al,Zn)49) for both grain refinement and age-hardening. It deploys a uniform distribution of µm-sized T-phase particles, which can be dissolved upon final solution annealing, and which is utilized for heavy particle stimulated nucleation (PSN) during industrial processing of sheets, in order to reach an equiaxed grain size as low as 4 µm. This fine grain size is advantageous for high-temperature forming of aluminium alloys. Elongations above 200% and 400% are achieved when deformed at strain rates of 10−2 s−1 or 5*10−5 s−1 at 470 °C, and interestingly, the fine grain structure is highly stable even when held at that temperature for one day. Moreover, the material reached yield strength values of more than 380 MPa after a paint-bake heat treatment for quenching in water or compressed air. The study demonstrates the importance of PSN using electron microscopic and texture measurements and describes it by simple modelling of T-phase particle grain refinement. It explores the high stability of the fine grain assembly in terms of the random grain boundary misorientation distribution in combination with high solute content observed, provided via the dissolved T-phase, and Smith-Zener pinning. The simple, commercially available grain refinement strategy demonstrated using the dissolvable T-phase, and the resulting unique property profile, make the crossover alloy in question a promising candidate for high-temperature sheet forming processes.