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)

My vote is for a triarchy consisting of: a kid, middle-aged, and elderly. 

Could also be fun for the US to have a third legislature chamber based on this, aiming for more generational perspectives and long-term thinking.

Thought of this when first hearing about Audrey Tang’s idea of transcultural conservatism.