The Credit Card Accountability, Responsibility and Disclosure Act, or Credit CARD Act, mandates that while the account holder is younger than 21, the co-signer has to give written permission for any credit limit increases, says Chi Chi Wu, staff attorney with the National Consumer Law Center.

However, once the cardholder is past the age of 21, no federal law requires that the co-signer be notified of any credit line increases, she says.

You may be thinking of co-signing, believing that if anything goes wrong, you can just write a check for your student's relatively small card bill.

However, if your adult child keeps that account open past his or her 21st birthday, the bill could end up being quite a bit more than you estimated.