Does offside apply in 9v9, and where?

Yes. Offside comes in at Under 12 and applies in all 9v9 and 11v11 football. The FA's 9v9 laws use Law 11 of the Laws of the Game unchanged, so there is no special offside zone or line:

• A player is in an offside position if any part of their head, body or feet is in the opponents' half (not counting the halfway line) and nearer the opponents' goal line than both the ball and the second-last opponent.
• Being in an offside position isn't an offence in itself. It's only penalised if the player becomes involved in active play when the ball is played by a team-mate.
• There's no offside offence directly from a goal kick, throw-in or corner kick.
• The restart is an indirect free kick where the offence happened.

There's no offside in mini-soccer (Under 7-Under 11), so for many Under 12 players this will be new.