radeng, the rules as I understand them are:-
a, no children or infants to occupy seats designated as Emergency exit seats, which means no one under 12 on a child fare ticket.
b. no one with impaired movement i.e. stiff legs, bad backs etc
c. no one who is deemed, by check-in or the cabin crew, to be physically incapable of opening the exits i.e. old enough but frail
d. no wheelchair or physically or mentally disabled pax.
e. no pregnant women.
f. No unaccompanied minors
Each airline has seating charts which designate the rows and the computer systems shouldn't allow an infant or child to be seated in these rows. Manual check-in a different bal game.
A most enjoyable moment at check-in is when Mr Painintheneck insists that they have an exit seat because they have a problem with their leg which has to be kept extended. ' I'm sorry sir that cannot be done for safety reasons'