Theoretically when you are on the tube you are supposed to give up seats to the elderly (and indeed I do this).

However my grandmother (and I'm sure she isn't atypical of the elderly) is hugely restricted in how far she can walk and how long she can stand for the precise reason that she hasn't done enough exercise in the past 20 years and is paying for it now.

So by giving up seats to the elderly are you actually inadvertantly contributing to their inability to walk/stand for long periods?