Twenty-one or over, checked on both of you
Everyone present at a booking has to be twenty-one or over, and a photo document may be asked for at the door and is never copied or kept.
Whoever booked and everyone else in the room has to be twenty-one or over. On a booking for two people that means both of you, and it is checked at the door rather than assumed from a message.
How the check works
She may ask either or both of you for a photo document on the doorstep. It is read once and handed back in the same movement. Nothing is copied, photographed or written down anywhere afterwards, and no record of it exists once she has left.
What happens if she cannot settle it
She leaves before the session starts. The same is true if she cannot establish that both people in the room know about the booking and want it, which is a separate question from age and is asked for the same reason.
Both partners agreeing
A booking for two is arranged on the understanding that both of them know about it and both want it, which is why the first message is asked to say so plainly. Arranging it as a surprise for somebody who has not been told is not a booking the house will take. The age page puts the whole of it in one place.