There you are - being king of France is a good thing. But what if you're too young to be king?
You get a regent.
What is a regent?
A regent is the person who rules in your name. While you play as the young, underaged king of France you use the stats of your regent. If you have an intrigue skill of 20 and your regent has 5, then the game uses 5 as your ruler's intrigue rather than your 20. You can never take any hostile action against your regent unless an event is triggered in which they steal money from you and you can punish them. Regents are not chosen by you unless you are a ruler who designates a regent. As a child you have no choice.
Under the new system, diplomatic actions such as revocation of titles can only be done if the regent likes the decision, if you attempt to revoke a title of a character who the regent prefers over you, then you cannot revoke that title. Other decisions such as imprisonment, executions and sending vassals into exiles are also effected.
Choosing a regent
To choose a new regent, you must be a ruler of age, find a preferred male character, and open diplomacy selecting to grant the grant honorary title and then select designated regent. Always appoint one with the best average skills, but not necessarily the first as they may hate you. Avoid those who will become pretenders. Try and find a good friend of your ruler to be the regent as this minimizes the risk of assassination of your future character who is your heir succeeds as a minor. -slightly outdated-
Be warned the designated regent is designated regent for life and cannot be changed unless revoked. If the regent dies when your character is a minor and is regent, somebody at random is chosen.
Make sure the regent doesn't have a rivalry or other reason to hate your heir, or assassination is the likely outcome of an underaged ruler rivaled with a regent. Also, underage rulers cannot perform most actions without a regent's support, and a low relation will eliminate any chance of accomplishing anything.
Regents also cannot be replaced on the council until the ruler becomes of age, regardless of their skill. It is difficult to replace these councilors because they cannot be dismissed and oftentimes appoint them self to a job on the council, even if they have no skill in it. If a troublesome regent is on the council, assign dangerous missions such as studying technology, fabricating claims, spying, and collecting taxes.
Family can make good or bad regents, depending on how they are used. Avoid putting your other children as regents, as they will be jealous and cause problems for the heir. Close family like uncles, aunts, nieces, and siblings can be effective if all the close family gets along. Be wary of family with title claims or distant family, as distant family tends to have less land and be prone to jealousy and revolts.