Why is a broken hip so dangerous in old age?

The blood supply to the head of the femur runs through a narrow route, and a fracture in the wrong place can sever it, so the bone dies rather than heals. That is why some hip fractures are repaired with screws and others require the joint to be replaced outright - the decision is about whether the blood supply survived.

The wider danger is not the bone. It is what happens to an older body that stops moving: pneumonia from shallow breathing and lying still, blood clots, pressure injuries, muscle wasting that begins within days, and delirium. Mortality in the year after a hip fracture in the very old is high, often quoted around twenty to thirty per cent, and much of that is not the injury but the immobility.

Which is why the modern treatment is surgery within about 36 hours and getting the patient on their feet the next day, even when that seems counterintuitive. It is also why the fracture is usually a symptom rather than an accident - it indicates osteoporosis and a tendency to fall, both of which are treatable, and the strongest predictor of a hip fracture is having had a different fragility fracture already.