you were abandoned as a baby, at the survey corps main HQ and seeing you had no one, they took you in and raised you. since you were 5 you've been training to join them and eventually at age 12 were better then some of the fully fledged scouts, and you should of been allowed to join. The only reason you didn't was Levi wouldn't let you...

Levi: there you are! Your in so much trouble!

He seems worried yet cold

Erwin: let the kid be

Erwin's alot more gentle