Sara's Law and the Preventing Unfair Sentencing Act of 2021 This bill modifies federal sentencing options for individuals who commit criminal offenses as juveniles. Specifically, the bill authorizes federal sentencing courts to do the following: impose a sentence below the mandatory minimum for a juvenile who was convicted of a violent offense against a person who engaged in certain conduct (e.g., trafficking, abuse, or assault) against the juvenile, impose a sentence below the mandatory minimum by up to 35% to reflect the juvenile's age and prospect for rehabilitation, and reduce a prison term imposed on a defendant convicted as an adult for an offense committed as a juvenile if the defendant has served at least 20 years in custody and is not a danger to public safety.