Since the timeline for this series is so nonexistent… er, unclear, for the purposes of this fic I'm making Mokuba 13 and Rebecca 12.
Disclaimer: Totally not mine.
Thirteen is too old to be just friends with a girl, but Nii-sama has no time for romance and doesn't see why everyone else can't be the same way, and sometimes it seems as if Yuugi and the rest will always think of him as being about eight years old, so no one seems to mind him breaking that particular rule.
Thirteen is too be old to be just friends with a girl, but for a girl, twelve is too old to be just friends with a boy, so Rebecca's in the same boat. She doesn't seem to mind, though, and doesn't seem to care that twelve is too old for her to sit on his bed, their knees just barely touching, and argue with him about whether Japanese manga are better than American comics.
And if thirteen is old enough to find the feel of her knee against his more distracting than it should be, and old enough to wonder how hard she'll punch him if he tries to kiss her, it's also old enough to like knowing someone his own age who looks at him without seeing his brother standing over his shoulder, and it's old enough to know that people like that will become even rarer when he turns fourteen, or fifteen, or sixteen.
And thirteen is too old to be just friends with a girl, but it's also young enough to have time to wait before he possibly screws up a good thing. He's old enough to be vice president of a major corporation, and old enough to take care of Nii-sama, which is a full-time job in itself, but he's not too old to want to be just a little too young for something every once in awhile. For now he's old enough to look and to wonder, but not yet old enough to stop arguing about X-Men versus Yuu Yuu Hakusho, and lean over and kiss her.
That can wait until he's fourteen.