When I said "in development", I meant in the early to adolescent years. Almost without a doubt children would, statistically, benefit from a sex segregated primary and secondary schooling where curricula and environment would be more likely to be tailored to their psychological and physiological needs.What about in public schools, higher education, or in the Service Academies?
Further, during early to adolescent years, sex segregated environments allow youths to develop a sense of self and what their gender identity is, without the distraction or influence of the other. Boys working out what it means to be a man and girls working out what it means to be a woman, ideally with the guidance of the their elders.