Are there any regulations on who can sit next to emergency exits?
EU No 965/2012 provides some direction.


  CAT.OP.MPA.165 Passenger seating
  
  The operator shall establish procedures to ensure that passengers are seated where, in the event that an emergency evacuation is required, they are able to assist and not hinder evacuation of the aircraft.


Further clarification can be found from Acceptable Means of Compliance
(AMC) and Guidance Material (GM) to Part-CAT. 


  AMC2 CAT.OP.MPA.165 Passenger seating
  
  ACCESS TO EMERGENCY EXITS
  
  The following categories of passengers are among those who should not be allocated to, or directed to, seats that permit direct access to emergency exits:
  
  (a) passengers suffering from obvious physical or mental disability to the extent that they would have difficulty in moving quickly if asked to do so;
  
  (b) passengers who are either substantially blind or substantially deaf to the extent that they might not readily assimilate printed or verbal instructions given;
  
  (c) passengers who because of age or sickness are so frail that they have difficulty in moving quickly;
  
  (d) passengers who are so obese that they would have difficulty in moving quickly or reaching and passing through the adjacent emergency exit;
  
  (e) children (whether accompanied or not) and infants;
  
  (f) deportees, inadmissible passengers or persons in custody; and
  
  (g) passengers with animals.


Whether or not a person can sit in the emergency exit row may also depend on if the specific seat is designated as an emergency exit. For example, on narrow body aircraft, generally all seats in the emergency exit row are considered emergency exit seats and only persons who "able to assist" can sit in those locations. However, on some wide body aircraft, only seats on the window sides are considered emergency exit seats. On an aircraft with a 2-3-2 configuration, the 3 seats in the middle are not considered exit row seats and therefore do not have any further safety requirements.