How old do you have to be to get into a PG movie without an a adult?

My friend and I (me 12 her 13) want to go see Diary of a wimpy kid this Friday. But apparently due to rude humor and language it is PG. Our parents are fine with us going alone but will we not be allowed in? what is the age restriction for PG anyways: we are both seventh graders and have student cards ( I realize that it does not affect what ticket you buy, but if they ask how old you are...) thanks in advance!