A law firm's desire to avoid having to expel partners by performance management in order to maintain a congenial and supportive culture can be a legitimate aim. But it will not justify setting a retirement age of 65 unless the firm can produce evidence that performance does in fact tail off at 65.
The firm here should have reviewed its past experience of underperformance or that of similarly placed firms in order to determine the appropriate retirement age to achieve this aim.
Employers will not always need concrete evidence to support their justification arguments. The tribunal was entitled to accept as a matter of common sense that knowing that partners would retire at a fixed age would assist in the retention of associates and in forward planning, two other legitimate aims put forward. But the aim of congeniality relied on a stereotypical ageist assumption that did therefore require supporting evidence.
The EAT also helpfully confirmed that employers can rely on legitimate aims they did not have at the time a rule was adopted. (Seldon v Clarkson, EAT)