He grins at her as he shrugs. "You're younger than me."

It was a habit. He called just about everyone who was younger than him 'kid' at some point. She was probably in her twenties just judging by her appearance. Unless she was one of those super slow aging alien races he figured she was younger.

"But I'll cut it out if you're bothered by it." Cobb doesn't want to insult her. It's actually something he does when he likes someone.

Merrin's clearly out of place here and trying her best. It's endearing in a way. He wants to help her get comfortable and confident. She's probably a force of nature when she's confident, he imagines.