How far back from the batter should the cacther set up?

My young catcher(8-9) has a habit of ending up on his knees a lot. I think it is because he is too far back and is having to fall into the ball to get to it before it hits the ground. Think nine year old pitchers in fall ball without a lot of velocity.

It's not easy getting a young gun to set up very close to the batter. I want to make sure I am giving him proper advice.

Thanks in advance,

Tim

Once catcher is in stance, the rule of thumb is usually an arms length away from hitters back knee.