This is what volume 1000 says about Kuzan: A monster recruit who doesn't even
need to be taught (just getting scolded)
Akainu, also known as Sakazuki, enlisted in the army two years before Kuzan, and although his age is slightly different, he is also called a monster recruit just like Kuzan.
He enlisted in the Navy 30 years ago at the age of 18. At that time, he
was called a "monster recruit" and was taught from the beginning.