Twenty-one or over, and that means both of you
Everyone in the room has to be twenty-one or over, the partner who wrote and the partner who did not, and a document may be asked for at the door.
The rule is one sentence long and it has no exceptions. Twenty-one or over, everyone present, whoever booked and whoever came along. It applies to the person who opens the door and to the person still in the shower.
At the door
She may ask to see a photo document from either of you. It is looked at once and handed straight back in the same movement. No copy is taken, no photograph of it, no scan, and nothing from it is written down anywhere. The age statement sets out the whole of it.
If the question cannot be settled
She leaves before the session begins. That is not a judgement about anybody and it is not open to discussion on the doorstep. The fee for the reserved time stands in that case, for the same reason it stands whenever a booked hour cannot be given to anyone else.
Why twenty-one rather than a lower figure
Because the house has set its own line there and keeps it everywhere, on every booking and in every room, without regard to what any other threshold might allow. It is a single number that is easy to state in writing before a trip and easy to check at a door.
Where to say it
Nowhere in particular: it is assumed rather than asked. If one of you would rather have the document question out of the way before she arrives, say so in the thread and it will be confirmed in writing, which some couples prefer when the room is shared and the evening is short.