15+ years old, not traveling with child or with in-cabin pet, physically able to operate the door, willing and able to understand and follow crew member instructions.

Typically, the exit row briefing comes before the plane leaves the gate, so if there are going to be seat jumpers, the sooner the better. If they switch late, then the FA's have to repeat the briefing.

If their luggage is behind them, that's their own problem to deal with...