The AlMgZn phase (T-phase)-an interesting strengthening phase in precipitation-hardened aluminum alloys has not received sufficient attention and is very different from the commonly studied η-MgZn₂ phase. This study focuses on Al-5.1Mg-0.15Cu-xZn alloys with a Zn/Mg ratio below 1.0. This is different from commonly known 7xxx series alloys whose Zn/Mg ratios are above 2.0. The relations among the age-hardening response behavior, clustering of Mg-Zn or Mg-Cu atoms, and precipitation of different hardening phases in Al-5.1Mg-0.15Cu-xZn alloys are investigated via atom probe tomography and transmission electron microscopy. According to the results, the Zn addition stimulates the precipitation of coherent T″ precipitates, but suppresses S-Al₂CuMg and β-Al₃Mg₂ phases. This results in an enhanced and accelerated age-hardening response in alloys with high Zn contents. The 3.0Zn (wt.%) alloy with finer T″ precipitates exhibits the best age-hardening response, followed by the 2.0Zn (wt.%) alloy with T′ precipitates and the 1.0Zn (wt.%) alloy with T precipitates. Clusters with high Mg/(Al + Zn + Cu) ratios cannot act as effective precursors for the transformation of Mg-Zn clusters into T″ precipitates, thereby resulting in a weaker age-hardening response and lower hardness in the 1.0Zn (wt.%) alloy. Furthermore, although T-phases originate from different alloys, their Mg/(Al + Zn + Cu) ratios reach the constant value 3/7 for larger particle sizes. This challenges the chemical composition of traditional equilibrium T precipitates.