Seat Allocation
To expand on jalguy, from a CAA Safety Publication:
"The CAA requires that UK airlines have procedures to ensure that passengers are seated where, in the event that an emergency evacuation is required, they may best assist and not hinder evacuation from the aeroplane.
Only those passengers who appear reasonably fit, strong and able to assist the rapid evacuation of the aeroplane in an emergency should be allocated seats which permit direct access to emergency exits. Persons of reduced mobility should not be allocated seats where their presence could impede the crew in their duties, obstruct access to emergency equipment or impede the emergency evacuation of the aeroplane.
The following categories of passengers are among those who should not be allocated to, or directed to, seats which permit direct access to emergency exits :
Passengers suffering from obvious physical, or mental handicap to the extent that they would have difficulty in moving quickly if asked to do so.
Passengers who are either substantially blind or substantially deaf to the extent that they might not readily assimilate printed or verbal instructions given.
Passengers who because of age or sickness are so frail that they have difficult in moving quickly.
Passengers who are so obese that they would have difficulty in moving quickly or reaching and passing through the adjacent emergency exit.
Children (whether accompanied or not) and infants.
Deportees or prisoners in custody.
Passengers with animals.