Twenty-one or over, with a passport to hand
Every booking is for people aged twenty-one or over: the guest who booked and anyone else in the room, confirmed at the age gate and, if asked, at the door.
The age rule on this site is twenty-one or over, and it covers everyone present, not only the person who wrote the message. It is confirmed twice. The first time is the age gate you pass before the site opens. The second can come at the door, where the therapist is free to ask for a photo document before anything begins.
What visitors should carry
A passport serves perfectly well, and so does a national identity card or any other official document with a photograph and a date of birth. She reads the date and hands the document straight back. Nothing is copied, photographed or noted down. Travellers sometimes leave passports in a hotel safe, which is fine as long as the safe is in the same room.
If there is any doubt
If she cannot be satisfied that everyone in the room is twenty-one or over, she leaves before any session starts. That is not a matter for discussion on the night, and it is why a couple should mention in the first message that two people will be present. The age statement sets the rule out formally, and the terms place it alongside the rest of the agreement.