There could have been a sudden death of the previous king/queen, who left behind no legitimate heir, so that the country had to be scoured for a bastard child, or a cousin, or a remote relative. If the priesthood was on top of things, they could have even murdered the king/queen, when they were ready to have their puppet put on the throne, and then after a good amount of searching had elapsed, present her as the fully-fledged 30-year-old that is the best option around. They might even pick the look of someone that actually existed and murder them too to cover their tracks. (Of course if the priesthood is that good, it might be difficult to get away from them/dismantle them)

But yeah, if I was writing a feature-length story, I might change the age, or work out the plot holes a little more in the edits so that it is easier to enjoy!

Cheers, and once again thanks for reading!