with the airline i work for, the passengers can pay extra for the exit row seats at the timeof booking their holiday. this can be done online also. however, every passenger who has paid or is allocated an exit row seat has to agree that they are 14+ years of age and physically and mentally able. it has happened in the past where people have paid for these seats and they are not physically or mentally able. they will be moved. it does not matter how much they have paid for the seats, they will not sit there. and they won't get a refund either.