People in lung cancer screening are often at high risk of dying from causes other than lung cancer, which limits how much screening can help them. Using CT scans from over 12,000 participants in the SUMMIT lung cancer screening study, we automatically measured signs of ageing in bone, muscle, blood vessels and body fat, and combined them to estimate each person’s biological age. People whose bodies looked older than their actual age were at much higher risk of dying from causes other than lung cancer. Because these scans are already taken for screening, biological age could help tailor screening decisions to each person’s overall health.