So there could be a few reasons for that.

While sex acts between adults and juveniles (under 18 years old, and previously under 20 years old) were illegal for a long time, the pornography thing is still a relatively new law, only a few years old.

Akagi’s story somewhat continues in Fukumoto’s other manga, Ten. The Akagi manga is technically a spinoff. It was really surprising the manga didn’t end several chapters ago when the 20 year long Washizu duel concluded.