I remember in "Once Upon a Time on Mars," Rimfire says, "I'm 16 and still sitting at the kids' table."

Yeah i know but that happen years ago, that was quite before the trio went to the earth for first time nwn
So yeah, he was really younger. But after that, he looks all grown up when the trio went back to Mars in the episodes "Back to Mars", even Modo notice that when he sees his nephew in those episodes, Even in "Stalkers" and "Caveat Mentor" Rimfire looks more like an adult.
So that's why i think he was like 18 or 19 years old on those episodes nwn

Okay, that makes sense. "Stalkers" especially was a fantastic episode.