At what age should boys not be allowed to use the women's restroom?

Cocoa Puffs • Psychology major 4.0 💕 Follower of Jesus Christ and God 📿 Married to my best friend and first love💍

I was at Wal-Mart, and this woman had her nine year old son with her in the bathroom. I didn't think it was that much of a big deal, but other women were giving the mom dirty looks. The question is at what age should boys not be allowed to use the woman's restroom? What about girls? Should there be an age limit for a child to use the opposite sex bathroom at all?