If it is decided through the authority of the PIC that on safety grounds your seat is to be changed then no argument. Discrimination, age or otherwise doesn't come in to it.
What absolute nonsense.
The pilot must be able to justify and substantiate his decision with a logical, probative justification for their decision. Just because someone is a pilot, does not mean all other laws get over-ridden. You can grope a passenger and say there is no argument, because it is on safety grounds and the PIC has authority? Do you know how old the passenger is or you just guessing they can't open the door? It is concerning the ignorance about authority that is sometimes displayed on this forum. You might want to hide behind safety as an excuse, but if someone takes you on, watch out.