Now that is a good question. From the IRS: Generally, the IRS can include returns filed within the last three years in an audit. Additional years can be added

Now that is a good question. From the IRS: Generally, the IRS can include returns filed within the last three years in an audit. Additional years can be added if a substantial error is identified. Generally, if a substantial error is identified, the IRS will not go back more than the last six years.