Hmmm. GOOD QUESTION.

Nine, clearly. "Stupid apes, poking your heads into every corner of the universe without reading the danger signs."

Rose: "Doctor, are you talking about him or you?"

Or, well. Ten would work. "Oh, hello there! Sorry for popping in like that."(ARE YOU BETTER WITH NINE OR TEN?)

Or Luz is running from bullets and sees the police box and dives for it without really knowing wtf a police box is doing in the middle of the Ardennes. STUFF HAPPENS.