something to call someone who's is worldly but still immature. Originated with the Doctor Who fandom, calling the Doctor one. He knows a vast amount of information on the universe, but still is young at heart, and has a tendency to be immature.
DW fan 1: Did you see that part where he made the angel say "comfy chairs"?
DW fan 2: Oh yeah, what a cosmic nine year old
Yeah, I mean he's nice and smart and whatever, but he's still not grown up, he's just a cosmic nine year old