Twenty-one or over applies to every client and to everyone in the room, and the age gate is the click a visitor makes to say so before reading on.
A strip along the foot of the first page asks one thing: are you twenty-one or over? Click to confirm and the strip steps aside. That click is the age gate. It is a statement the visitor makes about himself, and the page keeps reading beneath it the whole time.
Bookings are taken only from people who are twenty-one or over, and the same holds for anyone else in the room during a session. The message that asks for a booking is the client's own word that this is true of him and of everyone who will be there. Every woman listed on the roster is twenty-one or over as well.
Anyone can click. The rule travels with the booking and reaches as far as the door of the room. At the door, your therapist may ask for a look at photo identification. She glances at it and gives it straight back: nothing is copied, scanned or photographed, and not a word from it is noted down. If she cannot satisfy herself that everyone present is twenty-one or over, she leaves before the session starts, and the visit ends there.
A gift, a birthday, a stag weekend with a friend who looks young for his age. The line applies to the person in the room, never to the person who paid. Book for a twenty-year-old and there is no booking.
The pages behind it are written for adult readers, in plain words. The strip keeps them from opening in front of someone they were never meant for, whatever search result brought that person here. It also states the house line before the site says anything else. Your answer is remembered only on your own device, so the question does not come back on every page.
The rule is set out on the age page, and related questions sit in the FAQ.