Sometimes you will see something like the following (from American at http://www.aa.com/i18n/agency/Bookin...p&locale=es_ES

Qualifications

Customers must be advised that government regulations prohibit seating passengers in designated exit seat, on any commercial flight, when they answer NO to the following question. "The government requires any passengers seated in the exit seat be 15 years of age or older, be willing to assist in an evacuation and have no physical impairments that would prohibit your assistance. Airport personnel will make the final determination. Do you meet this criteria?"

Detailed Description

dot dot dot

Are you willing to assist on an evacuation?

Do you lack mobility, strength and dexterity in both arms, both hands and/or both legs to: quickly reach the exit, remove any obstructions, manipulate the exit door and slide mechanisms or lift out a window exit door, exit the aircraft, or assist others in exiting?

saying you need to be willing to assist. Though of course, many people will think assisting is opening the door and getting the heck out of the way so others can get out