@Mojo well I did say in my comment the ML is 15, so he's also underaged not just FL. Also, not just feudal times but even now anyone underaged can get married with guardian consent. Although they're married, their relationship is very akin to middle school/high school romance, it's a pretty innocent/first love type of relationship.
i feel like it would've been better if the author cut out the reincarnation part. it should've been a story without a grown adult reincarnating, acting like an actual child. i honestly actually expected her to be like this once i saw how she looked like in ep 0. she looked too innocent and as expected her personality is as well, sad but i hope she becomes stronger later on :)