Twenty-one or over, however long the stay
Everyone who books, and everyone present at the address, must be twenty-one or over, and a photo document may be asked for at the door on any visit.
The age rule has no exceptions and does not relax with familiarity. Everyone who makes a booking, and everyone at the address during the visit, has to be twenty-one or over.
At the door
She may ask to see a photo document when she arrives. It is looked at and handed back straight away. Nothing from it is copied, photographed or noted anywhere.
On a long stay
A guest who has booked ten times may still be asked. That is not a lapse of memory. It is most likely on a visit where someone new is in the flat: a partner visiting from home, a friend staying over, a flatmate. If anyone other than the guest will be present, say so in the first message. A visit where the question of who is present cannot be settled ends before it begins.
Why twenty-one
The house holds this line on every booking, with no exception for a regular. The full statement is on the age page.