Monarchies should skip a generation (so, go from grandparent to grandchild). That way you reliably get long periods of fresh blood, as opposed to transitioning from a 95 year old to a 70 year old.

(Alternatively, abdicate early Akihito-style)

I think it's better to just do something similar to how the saudis do it by slowly transitioning power to the heir (crown prince) from the king as the heir ages. It allows younger blood to take on more responsibilities as they age while still preventing a child from having absolute power.