"Careful, I bite!" or "Do you bite?" or any variation thereof on the topic of biting. Be sure and say it like it's a good joke, and don't stop making eye contact until your target gives you a satisfactory response. If they mumble something and drop their gaze, you have won. (If they look you squarely in the eye and say, "I would literally cut you up and eat you," with perfect deadpan, you have lost.)
"Are you having fun?" Using this when you catch someone younger than you performing a menial task gives you an aura of material superiority, for you have just reminded them that not everyone has to make copies for a living. There is power in asking questions you already know the answers to, so go ahead and smile to yourself as they affirm that they are, indeed, having fun.
Give someone a compliment by addressing it to their parent standing right next to them. Rave about how [insert adjective here] this person is, but only while pretending you don't see them. She's so grown-up! is my personal favorite, because there is nothing that makes you feel less like an adult than having to awkwardly smile at people who are talking about you, but not to you.
Refer to them as "kid." This one is usually reserved for people who are only a little bit younger than you, but for whom you would like to increase the perceived age gap. A good rule of thumb is, Is this a term of endearment I would use for my young nephew? It is? Excellent. Make an effort to start calling them things like "champ" or "buddy" at the nearest possible opportunity.
"Honey" is sometimes a really nice thing to hear from someone you don't eat family dinners with- for example, a warm and wrinkled black grandma who you wish you were related to. There is a fine line between what is patronizing and what is straight-up charming– so if you ooze too much kindness, I'd stay away from this one. If not, fair game.
Honorable mention: the nonverbal Head Pat. This one is a winner because for the recipient, it combines the experiences of being both a puppy dog AND a floor-sitter in Duck, Duck, Goose. When properly executed, the effect is classic.
Two extra points if you mess up their hair, and ten if it was up in a bun.