Inherited NPC Domains and Experience Levels

A wise ruler might grant their heir a smaller domain to rule while they still live, so they can get accustomed to rulership ahead of time. That way, the gap to reaching king status is a bit smaller.

The British royal family actually does this. The heir apparent holds the title of Prince of Wales. On top of that Prince Charles is Duke of Cornwall, and Prince William is Duke of Cambridge. Though I have to imagine that Charles reaches his XP cap quite some time ago.

I don't know that being born into nobility is necessarily going to require one to rely on Domain XP to level up, generally speaking being the next-in-line-for-top-dawgage leads to an annoyingly interesting life. Examples of noblemen who nontheless could have gained 9th+ level by their own hand I'd point you to Richard the Lion Hearted, Napoléon Bonaparte, and Vlad Tepes.