It's spelled out unequivocally in FAR 121.585.
Like some other FARs, some crews routinely ignore it.
(b) No certificate holder may seat a person in a seat affected by this section if the certificate holder determines that it is likely that the person would be unable to perform one or more of the applicable functions listed in paragraph (d) of this section because--
(1) The person lacks sufficient mobility, strength, or dexterity in both arms and hands, and both legs:
(i) To reach upward, sideways, and downward to the location of emergency exit and exit-slide operating mechanisms;
(ii) To grasp and push, pull, turn, or otherwise manipulate those mechanisms;
(iii) To push, shove, pull, or otherwise open emergency exits;
(iv) To lift out, hold, deposit on nearby seats, or maneuver over the seatbacks to the next row objects the size and weight of over-wing window exit doors;
(v) To remove obstructions similar in size and weight to over-wing exit doors;
(vi) To reach the emergency exit expeditiously;
(vii) To maintain balance while removing obstructions;
(viii) To exit expeditiously;
(ix) To stabilize an escape slide after deployment; or
(x) To assist others in getting off an escape slide;
(2) The person is less than 15 years of age or lacks the capacity to perform one or more of the applicable functions listed in paragraph (d) of this section without the assistance of an adult companion, parent, or other relative;
(3) The person lacks the ability to read and understand instructions required by this section and related to emergency evacuation provided by the certificate holder in printed or graphic form or the ability to understand oral crew commands.
(4) The person lacks sufficient visual capacity to perform one or more of the applicable functions in paragraph (d) of this section without the assistance of visual aids beyond contact lenses or eyeglasses;
(5) The person lacks sufficient aural capacity to hear and understand instructions shouted by flight attendants, without assistance beyond a hearing aid;
(6) The person lacks the ability adequately to impart information orally to other passengers; or,
(7) The person has:
(i) A condition or responsibilities, such as caring for small children, that might prevent the person from performing one or more of the applicable functions listed in paragraph (d) of this section; or
(ii) A condition that might cause the person harm if he or she performs one or more of the applicable functions listed in paragraph (d) of this section.