Valid criticism but unfortunately it's what we have to deal with in historical fiction. In historical societies, you should get engaged and married fairly early. Engaged at 15 isn't that strange, some societies even going down to 13 (which is so ew), especially in an arranged marriage where the children have no say. I think it would be more out of the ordinary considering the setting, if the fmc was 20+ since despite her good family, she should've already had good marriage prospects and maybe even be considered 'unmarriable' due to her age. If it makes you feel better, from memory, the ML is 17 (but correct me if I'm wrong) so the age gap isn't a crime. Also they aren't engaging in intimate behaviors until the fmc is an adult which is good in my eyes.
HAHAHAHA allg like I said you're not wrong, cause your concerns def do happen! I too have given many side-eyes to some authors' age-decisions in manga and fiction in general... Give us back adult romance... I also enjoy this story, but I also have some concerns over certain artistic choices in it like you XD

like i wished i was in the bisexual cuck chair seeing them just railing each other in both forms of the fmc like i need this to happen so bad yall i love when the romance is giving bisexual