How?? Aileen behaves like a teenager girl and sejun is late 20s I think , she might not be a teenager but many readers already expressed that they would like for the story to focus on sejun and his farming and not on a more romantic plot with Aileen ofc it's the authors choice on how the story will follow and ofc this is my opinion and you have a different one
Aileen has human form, she’s 200+ years old, she behaves like that because for dragons she’s still considered as child but she is matured enough, the author is just hinting the romance between them but will not likely be together soon maybe it will happen after a year or two, and lastly the reader’s opinion is valid but the author already planned the plot so it is hard to change it we just need to accept that there’s a possibility that they’ll be together and not siblings/bestfriend relationship. This is facts + my opinion, thanks.

Finally a touching meeting (not really) between father-in-law and future son-in-law (⌒▽⌒)