Sorry to be late responding here... real life got in the way. Yes, the decision to make a romantic pairing between the 14 year old Crown Prince and 24 year old Oldest Maid Ha Na was very uncomfortable for many viewers, me included. Watching actors who have a real-life age gap of 17 years (he is 12, she is 29) just made it worse.
The writers did not have to include it in this drama which is a fantasy rom com using historical backdrop with a fictional royal family, not a straight saeguk. Following history isn't necessary. This particular subplot could have been handled differently. At the very least, they could have aged up the CP a few years. Make him 17-18 to her 24. At least then he would not be a minor. The decision to promote a pairing of a minor and an adult was for me a writing fail in an otherwise enjoyable series.
It was interesting to me that even the people who were okay with this pairing (Saying that marriages between minors and adults happened in the Joseon era and they accepted this pairing within that context) were making excuses in their own heads, saying that even though the CP-Ha Na were married, the marriage wouldn't be consummated until CP was "older." So, in my opinion, actually nobody liked this subplot which was totally unnecessary.
Overall, this drama was okay.
Things I liked: I liked the comedy, characters had depth, villain was a layered character, happy ending for everyone, including the old batchelors who were not chosen and the friend with the daughter. Sweet romance between the two leads, and cute moments for the other couples.
Things I didn't like: Creepy and unnecessary pairing of a minor and adult subplot. Drama had a slow pace and was draggy and boring at times, palace politics/mystery elements were confusing at times, as were the relationships between some characters, disappointing kiss scenes between main leads, the ending, while happy, was a little unsatisfying.
I don't regret watching this drama, but I don't think I will be rewatching it over and over again. It was more of a "watch it once" drama for me.