debt shows up on my credit report, I co-signed, am I responsible

The pupose of "co-signing" is to make you equally responsible for the debt.  It does not matter than they did not send you a demand letter.  The Creditor may not know how to contact you, or more likely, the creditor simply does not bother actively trying to collect debts for defaulted apartments, because the chances of collecting anything is very low.  (If someone could not afford to pay his or her rent, the odds are not good that the person can pay the debt later.)  So they probably just put the entry on your credit report hoping that you will pay it off if you want to obtain credit later.