They do look a tad too young. I assume it's just the art style. You probably have more of a problem with the male character looking that way. Cause I don't see any problem with the female character
If you showed me the cover pic of this manga with no context I would've thought this was an onee x shota story
Not terribly fond of the englishized title. imo it was phrased pretty badly. If I were to translate it, I would probably phrase it as,
"My Girlfriend/This girl, who has too much sexual desire is troubling me"
Or
"I'm troubled by my girlfriend/this girl who has too much sexual desire"