Virgin Frequent flyer

I have tried to search for this topic but for some reason it is not working so apologies if it has already been answered.
I have just tried to register my daughter for her Virgin frequent flyer card but it says that under 18s cannot be registered. She flies out the day before her 18th so I take it she will be able to register and claim for the return leg of the trip but would then only get half the mileage.
My question is this.
If she cannot become a frequent flyer until she is 18, how come she has to pay full fare? If she is paying an adult fare, she should qualify for the adult benefits. Surely airlines can`t have it both ways.[n]
We have a BA family card and we all register miles into a group pot of miles when we fly.

[RichardMannion]

She can join FC, but you cannot join online as it requires a parent or guardian to sign/agree to the terms and conditions. FC can send out a form for her, or you could have her join upon her return and retro claim the miles.

FC members have to be over 12 to join, unless they are part of a Gold account where they can join over the age of 2.

Thanks,
Richard

Thanks for the info Richard.