That's a huge interrogation for me, because I still don't understand fully the American system.
In France it would be 10+ unless supervised by adult. I think it would be teen or +15 in American standards?
There is non-sexual nudity and alcohol/drugs. Some scenes occur in a brothel, although there is nothing NSFW shown. No significant violence. So I'm not sure. What would you say?
My target audience, I suppose, would be adults anyway, even if it's a coming of age story, because it's pretty nostalgic. It probably appeals more to adults remembering their youth than to people experiementing teenage at the moment they read.
For now, though, it's probably G