The law:
General filing requirement
The IRS has general filing requirements for most taxpayers. Even if no tax is owed, most people file a return if their gross income is more than the automatic deductions for the year. The primary automatic deduction is the the standard deduction. Its amount will depend on your filing status and age. Standard deduction amounts change each year, however, a single filer’s standard deduction is always less than the amount a head of household filer can take.
Not sure how it works in the UK, but 50% of one's Social Security is taxable income, though most retiree's income is below the threshold where taxes would be due on it.
Are UK pensions considered taxable income?