Eighteen. Arthur's eyebrows jump slightly and he looks at her face, considering this new information.

"I'm nearly thirty," he tells her, and leaves it at that. He knows that probably doesn't pose much of a problem for her, but it definitely poses a problem for him - eleven years is a big difference whether one person is a teenager or not. However, he doesn't want to be rude, and part of him doesn't want her to move on to the next guy who'll probably be a creep and won't care that she's basically still a child.

He takes another drink.