It would have been easier for check-in to have sat the lady in another seat.
Once the lady is sitting there the cabin crew don't really have many options to move her. Cabin crew are not doctors or physio and hold not qualification or authority to tell the lady she is not capable of lifting the exit or performing the function.
What I do in this instance is just ask older people if they would be able to open the exit QUICKLY and get out QUICKLY to allow everyone behind them to do the same. I place emphasis on the importance in the role.
About half the time they choose to move and half the time they are eager to volunteer. At my airline we place all hand luggage in lockers at exits rows. Alot of the time little old ladies are not willing to part with their handbag and ask for another seat anyway.
I think the above is the politest way to ascertain as to whether or not an older person is willing and capable of opening the exit.
Cabin crew are in no position to tell someone they cannot perform the function based how fit they think they may be. I cringe every time I see a flight attendant try to tell someone they are too old to sit at an exit. There is more than one way to skin a cat as they say, and there is always a politer more PC way.