Debt can be kept forever. The original owner of the debt has the right to sell it to whomever they feel to try and collect. Sometimes they decide its not worth to try and collect and dispose of it, but if they don't it might be to your advantage to settle.
Normally NO. The creditors will ask for the clearance of their debts with a limited time period and they will write off as 'bad debts'. If it is a mortgaged debt then it will last for twelve years normally. The debtors usually change the address as they have not paid in time due to the loss of their job/business.
It depends upon the type of debt and, often, the state where you live. Debts remain unless a statute of limitations applies and eliminates them. The rules for this often vary by state. Some types of debts, however, will never go away if not paid. This includes some forms of tax debts, as well as student loans.
If there is no activity on the account like payment, it drops off the credit report in 7 years. Depending on where you live, there is a statute of limitations in which if they have not tried to collect, they cannot do so. This ranges from 4 to 7 years.