We do, but it seems most churches are not doing so biblically. Right now, we are in a "mixed" sunday school class.
The Bible says that the "elder" are to teach the "younger" and the "aged women" are to teach the "young women"? Why? Because "sober" (serious) grandpas and grandmas. for instance, have far more wisdom, maturity and life experience. You aren't going to get some of the stuff which needs to be taught any other way. For example: how well will a younger man, with ladies in his class, teach with any depth at all on "being obedient to their husbands" or "defrauding not"? Not very well.
Titus2
But speak thou the things which become sound doctrine:
2 That the aged men be sober, grave, temperate, sound in faith, in charity, in patience.
3 The aged women likewise, that they be in behaviour as becometh holiness, not false accusers, not given to much wine, teachers of good things;
4 That they may teach the young women to be sober, to love their husbands, to love their children,
5 To be discreet, chaste, keepers at home, good, obedient to their own husbands, that the word of God be not blasphemed.
6 Young men likewise exhort to be sober minded.
7 In all things shewing thyself a pattern of good works: in doctrine shewing uncorruptness, gravity, sincerity,
8 Sound speech, that cannot be condemned; that he that is of the contrary part may be ashamed, having no evil thing to say of you.