IDIOMS

be born yesterday

(idiomatic, informal) To be new, naive, innocent, inexperienced, or easily deceived.

Example:   I was not born yesterday, you know. I have done this before!
1915, William MacLeod Raine, Steve Yeager, ch. 6:
  "Say, do I look like I was born yesterday? See any green in my eye, Cactus Center?"
1998, Gwyn Hyman Rubio, Icy Sparks‎, page 155:
  "Compared to me, you were born yesterday."
2005, Howard Zinn and Donaldo Pereira Macedo, Howard Zinn on Democratic Education‎, page 69:
  If you don't know important things about history, then it's as if you were born yesterday.