In terms of the regulatory authorities there is a differentiation between an emergency exit and a ‘self help’ exit. Self help exits are the likes of the overwing exits on an Airbus A320 or a Boeing 737 where there are no crew present to operate them in the event of an emergency and passengers are given a briefing on how to open the exit and asked if they are willing to assist.
At exits where cabin crew are present the restrictions aren’t as tough. We don’t brief the passengers that they are sat near an emergency exit, are you willing to help in an emergency, this is how the door operates etc etc. Obviously there can be nobody or thing physically obstructing the exit – if someone is obese to the point of needing an extension seatbelt we’d likely move them. In terms of age – it’s a bit tougher. It wasn’t so long ago that at BA we had to retire at 55. Now, we can work until we are able to open that door on our recurrent safety course. So age is a difficult one to factor in.
In terms of a PRE PLANNED emergency (ie where we have time to brief passengers) we try and move those close to the exits that are fit, travelling alone, willing to assist and speaks english. In an unplanned emergency we’d hope that the cabin crew were able to open their door. If not – hopefully someone nearby watched the safety briefing!