The mechanical behaviour of sea ice evolves substantially as the ice ages, yet the mechanisms governing the associated changes in fracture resistance remain poorly understood. Here, we investigate the ageing of columnar saline ice through a multi-scale experimental fracture campaign combining controlled laboratory ageing, Arctic field measurements, and three-dimensional X-ray micro-computed tomography (&mu;CT). Laboratory-grown ice exhibits a pronounced increase in fracture toughness (<em>K_IC</em>) over 120 days of ageing, accompanied by an increase and eventual plateau in the work required. The fracture toughness of aged natural sea ice follows the established inverse relationship with the square root of brine volume, whereas the fracture behaviour of younger laboratory ice cannot be explained by brine volume alone. Instead, &mu;CT observations reveal microstructural evolution: dispersed brine inclusions coalesce into increasingly interconnected drainage channels with larger apertures, altering the geometry and spatial distribution of internal flaws. These observations suggest that this structural consolidation reduces local stress concentrations and forces crack propagation to transition from a highly splintered path to a singular, energy-intensive fracture, providing a physical explanation for the observed toughening. Together, these results demonstrate that ice age is a powerful predictive parameter for evaluating fracture resistance, while highlighting microstructural evolution as a fundamental physical mechanism driving this temporal toughening.