In the US, under age must have letter signed by other parent if traveling with only one as Luis stated above.
I don't believe it is as restrictive as to flight number, date, etc here though. Of course back in the 70's you could pretty much get a ticket for just about anyone of any age........you could pretty much put a kid on a flight alone.....no questions asked!! One could almost put a baggage tag on a child and away they went. My grandfather would pretty much do the same for us as kids on the train! The FA
may have been curious back then, but only to help out. WOW have things changed or what.
Mike @ MSP