Many of our clients contact us after having owed the IRS for 7, 8 or 9 years, often unaware their problem is close to over. The end date for IRS to collect an unpaid tax is 10 years from the date it was assessed. This is called a statute of limitations.
As to why you have not heard from the IRS in a while, we understand that the IRS has an Inventory Delivery System which ranks each delinquent account. The IRS risk scores take into consideration the amount of tax owed, the age of the account and the type of tax involved. Some types of tax delinquencies, such as trust fund employment taxes, are assigned a high risk score. Newer accounts are often assigned a higher risk factor than older accounts.