When seniors must file
If you are single and at least 65 years old and U.S. Social Security is the only income you receive, then you don't have to file a federal income tax return. But if you do earn other income , then you must file if the total income other than Social Security exceeds $11,850. If greater, then you have to file. Keep in mind that this income thresholds only apply to the 2015 tax year, and generally increase slightly each year.
When to include Social Security in gross income
There are certain situations when seniors must include their Social Security benefits in gross income. If you are married but file a separate tax return and live with your spouse at any time during the year, then all of your Social Security benefits are considered gross income which may require you to file a tax return.
In addition, a portion of your Social Security benefits are included in gross income, regardless of your filing status, in any year the sum of half your Social Security plus all other income, including tax-exempt interest, exceeds $25,000 or $32,000 if you are married filing jointly.