An organization has some members paying their fees annually by cash and therefore need reminders. There are three membership levels - Under 16, Over 65, and others. It's quite straightforward to set CiviCRM's membership types for this.
Other members pay by monthly direct debit, which means they continue to be members until they take action to cancel the direct debit. Bearing in mind they also are categorised by the same three levels - under 16, over 65, others - how should membership type be set up for these members?