Waiting until your king is 15 to retire him without paying GB can be painful for those with tighter budgets.
Consider a means for you to decide that your heir wants to challenge your current king, or attempt to prove himself worthy of taking over. You chose that your heir has decided his father's reign is over, he challenges and fights for the title. The king eventually loses and may be killed.
Or he attempts to impress him by demonstrating his suitability for king. By shows of stats, handsomeness, or mental capacity.
In order to succeed, it could take a few days of repeated attempts and your heir must be at least 3 years old.
The lore potential of having your heir earn his way to king

Suggestions are welcome!