WN has made the judgment, as it may do, that <=6 is a safety and security matter. Thus, seating together is assured. It has impliedly made the determination that above that, the child need not be seated with a responsible adult. The issue was debated in both Congress and at DOT and both determined that they would not intervene in carrier decisions.

As a passenger traveling with a child over six, you will in almost all circumstances find seats together, but are not assured of it. If you wish to be assured of it, you must purchase BS/EBCI.