Wine is sold to adults, and the delivery has to be handed to one. Cellar cannot check an age from a phone and does not pretend to. What it can do is make sure nobody is surprised at the door: the order carries a reminder that ID showing 21 or older is checked on delivery.
A confirmation checkbox is the usual answer, and it fails in both directions. It annoys every honest buyer and stops nobody. So the notice is part of the checkout summary instead, one plain sentence beside the delivery slot of each seller, in the normal text colour. It travels with the order to the winery, so the person who carries the box has read the same sentence.
The principle is to put a rule where the decision happens, in words a person would use. No warning colours, no modal, no icon of a stern hand. A neutral sentence, early enough to act on, respects the buyer and is clearer to the winery too.