Twenty-one or over, for everyone in the room
Every guest in the room, and every therapist, is twenty-one or over. She may ask for a passport or photo document at the door and hands it straight back.
Twenty-one is the minimum age for everyone involved. It applies to the guest who books and to anyone else present in the room, and everyone on the roster is twenty-one or over as well.
At the door
A therapist may ask to see a passport or another photo document when she arrives. In an airport hotel that is usually easy, because every guest has a passport within reach. She reads the date of birth and hands the document straight back. It is not copied, scanned or photographed, and nothing on it is written down.
If she cannot satisfy herself about everyone present, she leaves before the session begins. That is not a judgement about anyone; it is simply the rule applied as written.
Couples
For a couple, both guests must be twenty-one or over, and she may ask to see a document for each. Keep both passports somewhere easy to reach, not zipped into the bottom of a suitcase.
Why the threshold is twenty-one
The house chose twenty-one deliberately, and holds to it whatever age other businesses use. Nothing about the airport changes that.
Other guests in the room
The rule is about everyone present, not just the person who booked. If a friend or colleague is in the room when she arrives, the same applies to them, and she may ask to see their document too. If you are not expecting anyone else, there is nothing to think about. If you are, say so in the thread before she sets out, so there is no surprise at the door.
Where it is written
The age statement sets out the rule in full, and the terms refer to it. A guest who is twenty-one or over and books in his own name has nothing further to think about here.