Its so difficult to set a rule for them to retire. It seemed to me the time for the Queen was when her Mother died so that Charles could take over before he was too old. As an arbitiry number I would suggest 70 was a reasonable age to consider, this would have given Charles 20 years and William around 40..
You're setting a number against a specific scenario. That number will, in all probability, not work for the next generation of Royals. Rather than it be an arbitrary number, why not have a competency test. At least that way the person is employed based on their ability to do the job, rather than an ability to live a long time.