Once an employee is auto-enrolled with an Eligible status, their auto-enrolment status doesn’t change, so even if they reach State Pension age, or their earnings drop. You should continue to use their Eligible status when you submit their employee data.
But if the employee stops active membership and then asks to join again (or leaves and then returns to your employment), they should be reassessed based on their age and earnings.
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What happens when an Eligible employee reaches State Pension age?