Fix #12010: Use economy timer for vehicle stats minimum age, not calendar (#12142)

src/vehicle_gui_base.h

@@ -53,12 +53,12 @@ struct GUIVehicleGroup {
 		});
 	}
 
-	TimerGameCalendar::Date GetOldestVehicleAge() const
+	TimerGameEconomy::Date GetOldestVehicleAge() const
 	{
 		const Vehicle *oldest = *std::max_element(this->vehicles_begin, this->vehicles_end, [](const Vehicle *v_a, const Vehicle *v_b) {
-			return v_a->age < v_b->age;
+			return v_a->economy_age < v_b->economy_age;
 		});
-		return oldest->age;
+		return oldest->economy_age;
 	}
 };