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
A little piece a garbage that constantly screams into his mic and usually plays minecraft or call of duty black opps 3, also someone that older gamers troll to make viewers laugh.
Troller: *makes kid fall in lava on minecraft*
Raging nine year old: *earsplitting scream*
A nineyear old that breaks the sound barrier when they die in Fortnite.
Nine year old, Nine year old, here comes the Salty Nine Year Old, screams so loud, break the sound barrier, watch out! Here comes the Salty Nine Year old.