Twenty-one is the floor for every client and every therapist, and the age gate is the screen that puts that line in front of a visitor before anything else.
Before a single page opens, one question. Are you twenty-one or older? Answer it and the site lets you through. That screen is the age gate, and it is the first place the house minimum shows its face.
Plenty of adult sites settle for the ordinary age of adulthood. This house sets its line three years above that and holds everyone to it. Clients must be twenty-one. So must every one of the forty-five therapists. The distance above the usual mark is the whole idea: it leaves a margin, and a margin lets any doubt fall on the safe side.
Anyone can click a button. The rule does not live in the button. It lives in every booking the office accepts, and a booking that gives the office reason to doubt the line is met does not go ahead.
A gift, a birthday, a stag weekend with a friend who looks young for his age. The person in the room is the person the rule applies to, not the person who paid. Book for a twenty-year-old and there is no booking.
The pages behind it are written for adults, in plain words, about adult services. The gate keeps them from landing in front of someone who should not see them, whatever search result brought that person here. It also sets the tone early: this is a house with lines, and it states them before it states anything else.
The check is explained on the age verification page, and related questions sit in the FAQ.