K.A.
My daughter is only 14 months now but...when we get there..if she's got to go and with only my husband than the men's room it is until she's much older. I have 2 boys and my husband will often have me take them if they have to poop because men's rooms can be downright foul. It's not my first choice for my daughter to use the mens room for that reason but ya gotta do what ya gotta do. As for a certain age...I have a family member that was raped at 12 in a men's room so I personally am not comfortable with younger than that at all. A bit older than that at least. Safety before "modesty".