The age gate is the screen at the entrance to this site confirming the visitor is 21 or older; the house accepts reservations from adults of 21 and over only.
Every good hotel bar has someone who glances at the door. This site has a screen instead. Before any page loads, a visitor is asked to confirm that he is 21 or older, and continuing past it is his own declaration that he is. The text of that screen, and what it means, is set out on the age verification page.
Plenty of sites in this category settle for a lower number. The house chose 21 for everyone because a higher threshold leaves less room for doubt near the edge, where someone's own account of his age can be a year or two generous. It also matches the people who actually book from hotel rooms here: business travellers, conference delegates, couples on a long weekend and residents with an evening free.
The threshold is applied to the guest in the room, not only to whoever sends the first message. A reservation made on another person's behalf is held to the same standard, and one intended for someone under 21 is declined.
Most reservations need nothing beyond the screen and a sensible exchange in the thread. If something in that exchange suggests a guest may be younger than he says, the office declines rather than guessing. That is applied evenly and without any accusation attached, and the declined guest is not argued with.
The screen also does something quieter. A person who reached the site from a search result on a shared computer, or who handed a phone to a colleague for a moment, can close the tab before anything else appears. Hotel business centres and lobby terminals are exactly the kind of shared screen it has in mind.
How the site handles the little it stores about visitors is on the privacy page, and broader questions are answered in the FAQ.
Tonight
Name the room, the hour and how long you would like her there. The reply comes back in about ten minutes with who is free.