The age gate is the short confirmation a visitor meets before this site opens; every guest the house receives, and every therapist it sends, is 21 or older.
Some hotel lounges in Amsterdam admit only adults after a certain hour, and nobody finds that remarkable. This site applies a comparable rule at its entrance. Before the first page opens, a visitor is asked to confirm being 21 or older, and continuing is that visitor's own statement that it is so. The wording is on the age verification page.
Twenty-one is the line for everyone the house deals with. Every guest must have reached it, and so has every therapist among the forty-five. The house chose a figure above the common one because the margin removes ambiguity, and because it reflects the guests who actually write from Amsterdam's hotel rooms: people travelling for work, couples on an anniversary weekend, residents between engagements.
The threshold follows the reservation into the room. An assistant may place a request for an employer, and a partner may arrange one as a gift, but the person the therapist meets must be 21 or over. A reservation intended for anyone younger is declined without discussion.
For almost every reservation, the screen and an ordinary exchange in the thread are sufficient. If something in that exchange leaves the office uncertain about age, it declines rather than guesses. That is applied evenly to every guest, carries no suggestion of wrongdoing and is not debated afterwards.
The gate records a choice made on one device, not a profile of the person who made it. What little the site keeps about visits is described on the privacy page, and practical questions about a stay are answered in the FAQ.