No hard cc in the early game
http://euw.leagueoflegends.com/en/news/game-updates/patch/patch-718-notes
"With no hard crowd control in the early game, we want to make sure Lee Sin brings best-in-class 2v2 dueling power. We’re buffing up Safeguard’s early shield to make The Blind Monk’s perfectly-played ganks even more impactful."
So mid game begins when Lee hits level 6. K.
EDIT: Added source link