Twenty-one or over, for every guest in the room
Every guest who books, and anyone else present, must be twenty-one or over. A photo document may be asked for at the door, and a passport is fine for that.
The age rule on this site is twenty-one, and it applies to everyone in the room: the guest who made the booking and anyone else who is present when she arrives. There are no exceptions, and it is stated before a booking is made so that nobody learns it at the door.
The check at the door
Your therapist may ask to see a photo document when she arrives. For travellers, a passport is the obvious one, and it is likely to be within reach anyway after the flight. She looks at the date of birth and hands it back. It is not photographed, copied or scanned, and nothing from it is written down.
When she cannot be sure
If she cannot satisfy herself that everyone present is twenty-one or over, she leaves before the session begins. That is her decision, and the age statement sets out the rule in full.
Couples and friends
A booking for two, whether with one therapist or a therapist each, means two guests to satisfy the rule. Travellers booking together should both carry their passports into the room rather than leaving one in the checked case, which may still be on the belt.
Why twenty-one
The figure is a choice made by the house, set above the general age of adulthood on purpose. It applies on every booking, at every hour, at the airport and in the city, and it is part of what makes the arrangement straightforward for everyone involved. The terms state it too.