when should a 23 yr. old girl start paying rent

I live with my girlfriend, her 23 yr. old daughter and her mother. her daughter recently quit school for the second time and went to work in a car wash/oil change facility. she has credtit car debt of $15k & owes student loans $20k. evryone else in the house contributes to the rent and bills. I feel that since she made the decision to quit school and go to work she should also start to pay something. her mother says she can't afford it because she owes so much. I say that's not my problem. whose right