On UA 1K's certainly can pre-book exit row seats ( I have done this over 100 times over the past 5 years ) and UA certainly doesn't allow anyone under 18 years old in an exit row seat. On a UA 777 either row 16 or 17 if a First Suite-equipped plane and row 30 are the exit rows. You can view seating maps on most airline's web sites, incl UA.