And age gaps....
Like, why!?!
I see it in manhwas and manhua (see it straight romances), too. The whole in love with my master who took care of and trained me as a kid know is now my wife/husband. Will at least this guy didn't know and isn't trying to pursue him, the 16-year-old on the other hand will be trying.
Age gap isn't exclusive to only Japan. You'll see the trope in books, shows, dramas, comics, fanfiction, ect. For example, Games of Thrones, Vampire Academy, Married... With Children, Twilight, Glee, anything with historical (Anicent, fedural, victorian, medieval) settting, Eldorado, Pretty Little Liars, Buffy.
I didn't say it was exclusive.
Also most of what you sampled were either from a supernatural series or historical fantasy, which is par for the course given their nature.
An age gap of 5 to 7 years isn't something people balk at, it's the age of the person that does.
You said an age gap. I understand what you are saying (I do like how Skip Beat and Glass Mask were done). It's a popular trope in most types of fiction because ppl keep interacting with them. It is popular in Manhuas and Chinese novels; master/student.
Sidenote; I do know that people questioned the relationship between Vampire Academy; 16 and her mentor --- 20s. Also, Buffy who was 16 dating Angel.
Thinking about this manga made me realize isn't set in modern times from how the beastman or beast animal (?) dresses and is a supernatural/fantasy world. At least the guy didn't know and thought he he committed a crime. The 16-year-old is trying to pursue him.
Japanese people and their obsession with minors... Why...