How do I talk about retirement with an older employee without discriminating?

The answer Just Ask, HR gives

Supportive Sara — Warm and practical — people first, process close behind.

Your caution does you credit — and it's also slightly over-calibrated. The law removed the default retirement age, which means you cannot force retirement or treat someone less favourably because of age. It did not make the word "retirement" unsayable. The line is simpler than it feels: plans may be discussed; assumptions may not be acted on.

What you can lawfully do:

• Hold the same future-plans conversation you'd hold with anyone — ideally inside a regular appraisal framework for all staff: "Where do you see the next few years? Anything you'd like to change about your role?" Asked of everyone, it's succession planning. Asked only of the over-65s, it starts to look like something else — so build it into everyone's reviews.
• Listen to what "a few more years" might really be opening: many people want a glide path — fewer days, a mentoring role, handing the heavy lifting (literal, in a warehouse) to someone they train. Flexible working requests work in this direction too, and a phased handover is usually the best succession plan you could design.
• Plan succession openly and generally — developing a deputy is prudent management, not a hint.

What you must not do: repeatedly ask when they're retiring (that pattern reads as harassment related to age); nudge them out with worsened shifts or dropped responsibilities; assume the slowing you've noticed is age rather than something with a fixable cause.

On the "visibly slowing" point — treat it as you would at any age: if there's a genuine performance or safety concern, address it through your normal supportive performance route — a private conversation, an occupational health referral if health may be a factor, adjustments where they'd help. What you must never do is let concerns slide unaddressed because raising them feels awkward, then reach for retirement as the tidy exit. That's the pattern that ends in claims.

Held warmly and asked of everyone, the future-plans conversation is usually a relief to both sides. Most people know roughly what they want — they're waiting to be asked well.