A Pedagogical Question

If a school has co-education, when is the right time to end it? At the age of reason (between 7 and 8 years old), so second or third grades? Or at the beginning of puberty (which can be as low as 9 years of age if stressors are present, otherwise 11 to 13), 6 to 8 grade? Or somewhere in that range? It seems middle school (6 to 8) is a convenient point for dropping co-education, but perhaps it should be done at an earlier age, like at 9 years.