...author is just pulling your (and South Korea's law's) leg with characters' ages. Mentally subtract about 8 years from everyone's age, and it will start making much more sense.
Well, at least, as far as pornhwa about 12 year old kid going through puberty getting it on with every woman in his adopted family (and friend's sister) goes.

... Last updated on August 22nd, 2020, 3:27am

Anyone who reads this will notice that the MC isnt just ignorant towards sex. He doesnt behave in no way like a 20 years old young adult. Anyone can notice from little things like how he eats (he is the only person in his house who doesnt use chopsticks and holds the spoon as a toddler would) to how he interacts with other people that he is a "special kid".
The thing is he just happens to have a big pee pee and the adults around him had taught him in the worst possible way how to dead with his sexual urges.