How old? What class? Domestic or international? Regular pass of Family Vacation Pass?

For regular domestic travel, there really aren't any fees that I'm aware of, unless you are in First Class.

As far as age, not aware of any cutoff, but I'd guess they could go fairly young (9-10?) as long as they can follow instructions.

The risk is if they cause a disturbance, they could lose pass priveleges....or you could.