Why are middle-aged monarchs preferable to old ones?
What is the benefit of having a monarch who is merely middle-aged instead of one who is elderly? I mean, I don't see the point of a monarch at all. (The Netherland's royal family is, in my opinion, a foreign imposition--- the Netherlands have been a monarchy for less time than a republic.) So why does it matter that the younger members of the family will be crown princes or princesses for a long time? They are hardly likely to stage a coup in order to ascend to the throne.
If you want a young leader, elect one. If you insist on some hereditary ceremonial head of state, don't complain about the random and arbitrary nature of the succession. It's like complaining that the kings should have smaller ears or more hair or something.