Some programs have expiring miles (for instance, miles earned today will expire in 3 years if unused). Other programs have expiring accounts (accounts will become inactive if there’s no earning or redemption within 18 or 36 months). In either case, activity in your account can extend the life of your miles or in the extreme only some of your miles will expire on a given date.
One airline has had a policy far more draconian than either of those things.