Prior to the decision in Gross, it was common for Courts to give a mixed-motive jury instruction in age discrimination cases wherein the jury would be instructed that it should find for the Employee if age was a motivating factor in the negative employment action, and that age was considered a motivating factor if it played a part in the negative employment action.

Gross changed everything.  In Gross, the United States Supreme Court found that unlike Title VII of the Civil Rights Act, the ADEA has specific language that requires the "but-for" analysis even in a mixed motive case.

            As a result of this statutory interpretation, the U.S. Supreme Court held:

[T]hat a plaintiff bringing a disparate-treatment claim pursuant to the ADEA must prove, by a preponderance of the evidence, that age was the "but-for" cause of the challenged adverse employment action. The burden of persuasion does not shift to the employer to show that it would have taken the action regardless of age, even when a plaintiff has produced some evidence that age was one motivating factor in that decision.

Gross v. FBL Financial Services, Inc.  129 S.Ct. 2343, 2352 (U.S.,2009) [Emphasis Added]

            In sum, Gross states that the burden of persuasion never shifts to the employer. Instead, the Employee must prove the negative employment action was taken solely because of the employee's age.  Of course, the Employer still has the chance to present evidence disproving the employee's contention that age was the "but-for" cause of the discrimination.