Hi. This feeling of numbness/ coldness on sitting is most likely related to a decrease in blood flow due to pressure over the arteries carrying blood to your lower limbs. This happens as a normal phenomenon when anybody sits in a certain position for long durations without any change. In young individuals the tone of the muscle is good, and soft tissue bulk is also adequate to prevent this, though it does happen in them too, but to a much lesser extent. But in older individuals, as the tone of muscles is much less, soft tissue bulk is much less, there is a much more frequent compression which leads to these symptoms frequently. So the best way is to keep changing postures/ just stretch and move your legs one every 15 minutes. This will help. Thank you.