A 52-year-old postmenopausal woman from Delhi presents with a 3-month history of a painless, hard lump in the upper outer quadrant of her left breast. On examination, the mass is 2 cm, fixed to the chest wall, and there is skin dimpling. Axillary lymph nodes are palpable and firm. Mammography shows a dense, irregular mass with microcalcifications. Core needle biopsy reveals infiltrating ductal carcinoma (IDC) with intermediate nuclear grade and 8 mitoses per 10 high-power fields. Immunohistochemistry shows ER+, PR+, HER2 negative (1+ by IHC). What is the most important prognostic factor among those identified in this case?