Twenty-one or over, asked twice
Everyone who books and everyone else who will be in the room has to be twenty-one or over, and that is confirmed in writing and again at the door.
The threshold here is twenty-one, and it applies to the person who wrote the message and to anybody else who will be in the room when she arrives. There is no version of a booking where it does not.
First in the thread
Before an hour is held, the booking thread carries a plain confirmation that everyone present will be twenty-one or over. It takes one line and it is asked for every time, including from guests the house has looked after before. A booking that cannot carry that line does not get held.
Then at the door
She may ask to see a photograph document on the threshold, and she may ask it of anyone else in the room as well. She reads it once and returns it on the spot. It is not copied, not scanned, not photographed, and nothing from it is written down afterwards. Where she cannot settle to her own satisfaction who is in the room, she leaves before the session starts, and the booking is treated as ended at that point.
Why both
Because a line in a thread is a statement and a document at the door is a check, and the two do different jobs. Neither is a formality and neither is skipped on a busy evening or a quiet one. The age statement sets the whole of it out, and the terms say the same thing in flatter language.