Quite young, it seemed. Ariana Dumbledore looked younger, in the enchanted mirror that hung above the couch on the wall across from her, than her listed age. That Bellatrix didn't much care for children made dealing with the much easier. She did, however, manage to recall that children spooked easily. And surely a child of her heritage would have some ridiculously cheerful expectation. All frivolity and idleness, no sense of duty or obligation or responsibility. But some exercises were worthwhile.
Smoothing her expression into a pleasant sort of smile, Bellatrix looked up and the door swung open, seemingly of its own accord. It shouldn't be so terribly difficult. Bellatrix was hardly oblivious to the delicate way some people handled children. However, she was not about to get out of her seat for Albus Dumbledore's kin. It was entirely possible that Ariana had no idea who she was, and Bellatrix hoped to maintain that for the moment, so she was prepared to nonetheless play very nicely.
"Ariana," she said, her tone softer and warmer than it usually was. "Do come in. Would you like something to drink?"