The kingdom where rulers are young
Once upon a time, lies in a faraway place a medieval kingdom following a classic european hereditary monarchy, excepted on two peculiar points. First, the ruler could be equally as Queen or King, but that point I will not linger on it too much. Then, a custom which has lived for millenias make it so that, as soon as the heir reaches the young age of 12, they become the new ruler of the country. And this even if the old monarch is still able to reign. The old ruler becomes the advisor of the new one and keep their political contacts, but in the end, it's the new ruler who makes the call on important decisions and who is publicly shown as the head of the country.
The detailed inheritance code
I know you like it, so here are some details concerning how the inheritance is done :
Regarding other members of the family, the eldest sibling is chosen as the monarch, but their younger brothers/sisters don't take their role as they become of age. If the current monarch dies, the power goes to the parent if and only if there is no siblings older than 12 who can take the crown. In case said parent takes back the power, they are "temporary" monarch until such (or new) siblings are of age. Cousins and uncles/aunts are chosen as a last resort in the event there is no direct heir and no ruler anymore, in which case the one closest -but older- to the age of 12 is chosen (aside from political shenanigans you have in such really bad situations).
Still following? Good! Now the advisor part : The eldest monarch's parent, or -said differently grandparent- keeps the title of main advisor if they get the chance to see two coronations in their life. However, the new ruler's direct parent still has some words to say as a secondary advisor. In other words, while the youngest is the one with the power in hand, they can call either of their parent or grandparent for tips.
Finally, to give a rough idea of the powers each party has : The young king at its youngest has most if not all the executive power, but until they become older, rely some of their legislative power on their advisors, since they can understand better the implications of new laws. For the young ruler it becomes kinda way of accepting or refusing a suggested law, then altering it and eventually have a word on all as they grow older and wiser. The transition process can be really quick if they prove themselves to be quick-witted, or last up to 6 years if they're less knowledgeable. Usually, at the age of 15 they have all powers a king or queen has, though.
Phew, I think it covers most of the topic and there should be no hole. I hope. On to the issue.
I can easily predict that this tradition would reduces in general the time one monarch is in charge, to as early as 12-14 years if they have very quickly an off-spring. However, I have an hard time in dreaming how well the outcome would be, and what good things can be taken from such tradition on a political standing.
To give a comparison, I know that young rulers did exist : For instance, Louis XIV in France became officially a king at the age of 5, and while he didn't actually take much responsibility this young, he did eventually well enough to be called the "Roi-Soleil", or Sun King. In another country and time, Tutankhamun gained access to the power at the age of around 9. Yet, these cases are often unwanted cases where the predecessor died, and such events create unstability in the kingdom (like the Fronde rebellion that Louis XIV faced). If it was known and correctly announced and prepared, there would probably less ruckus, but still I have some doubts...
So what are the advantages (if any) that early power pass-on is standardized and favored, especially in the political games you have with your vassals and other neighboring kingdoms?