Look, I'd prefer if VNs finally took off from the high school setting and offered us MILFs and characters in their 20s, 30s, and so on, with more varied settings and cooler stories. It seems pretty stupid to me that most VNs are aimed to an adult public and still depict teenage characters, I understand that for shounen because that's the demographic, but not this.
I know of nukige (huh) with children about 8-10 but I've not played them.
I know there are people that do it at 15-16 but not everyone. Even though the most success I've had is with teenage girls, they're fickle as fuck, I suggest moving away from them entirely. There's an upcoming anime about a high school girl romancing a man in his 40s but I laugh it off, besides, how appropriate and totally not sexist, it's always the girl that's young and the man who's mature.