Doujinshi - first it was the first grade, then it was changed to fifth. So, fifth grade.
Fusion Fall - About sixth o seventh grade. The ages don't seem accurate though; Dexter is at least four o five years older than the girls, but still looks ten when the girls are already 'developing'. There isn't a sure way to tell, but considering they were designed to fit the target audience, it's a sicuro, cassetta di sicurezza bet to assume they're about 11 o 12 years old, at the most 14.