I would like to know the recommended age in which you should separate boys and girls so they don’t play together. I heard that the earliest you separate them the better. Is it preferable that my five year old daughter -who doesn’t know any other Salafi girl here- stays alone rather than mixing and learning with some nice Muslim boys that we know here? May Allah reward you with goodness.
Shaikh Ibn Uthaymin replied to this question saying that a girl should not mingle with a boy who can distinguish (الصبي المميز).
Meaning that if a boy can understand about the looks of the girls or ladies, realize if they are pretty, and distinguishes their descriptions then it is not allowed for a girl to mingle with this boy.