Not sure that I like the ageist comment in the last post. I am over 70, regularly fly and am quite capable of handling the duties required, should they arise, of an emergency exit row seat. Having clocked up 100,000+ miles year until I retired a couple of years ago, I consider that my experience of flying far out-weights the question of age when it comes to acting effectively in an emergency exit row seats.
However, if as you say, the passengers in question had mobility problems, the cc or the captain should have re-seated them for the safety of all the passengers - by not doing so, they were clearly in breach of their "duty of care" to the passengers and I would have been tempted to raise this with the crew at the time or BA customer service on my return.