Surviving Widow(er) Income Fair Treatment Act of 2021 or the SWIFT Act This bill modifies eligibility requirements and increases Social Security survivor benefits for widows, widowers, and surviving divorced spouses of a deceased worker. Specifically, the bill makes widow and widower's benefits available to disabled individuals at any age, whereas currently the benefits are limited to such individuals between ages 50 to 60. In addition, provisions reducing benefits in various instances, such as when a beneficiary is entitled to multiple types of benefits, shall not apply to widow and widower's benefits for individuals with disabilities. Additionally, the bill raises the upper age limit for determining whether a beneficiary has a child in his or her care from 16 to 18 years (or 19, if the child is a full-time elementary or secondary school student). An individual with a child in care may receive increased benefits in specified instances. The bill also increases various limits on widow and widower's benefits in certain instances and modifies the calculation for increased benefits for individuals who choose to delay receiving such benefits. For current beneficiaries, increases in Social Security benefits under this bill shall not affect the beneficiary's eligibility or benefit amount for any federal assistance program.