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I looked it up a bit. CERO C means it's for 15 and up, but CERO Z being for adults only. Most likely because in this game you play a role as a teenager so it's not really for younger people to play it.
Actually, your character graduated from high school, and apparently after completing the game 3 times, there is an extra scene where you and Ayane apparently had sex (though nothing was shown, but it was damn convincing enough since she is wearing a night shirt, hinting you and her had sex)
'When in 21st century America, act like the freaking Romans 2,000 years ago. Random counter-productive brutality for the win!'