I've witnessed this before by flight attendants not knowing their own rules.
This was on an express flight. The FA argued with a passenger and his 15-year-old daughter that she was not old enough to sit in the seat. She kept on insisting that they had to be "older" than 15. Finally, after a good 3-4 minutes of discussion, the father pulled out the safety briefing card, and showed it to the flight attendant.
The FA then changed her tune to say she did not believe the 15-year-old met the physical conditions to sit in the exit row, specifically that she was not fit enough to lift the door.
