My son got very firm about using the men's room rather than the ladies' at about age four. When we are out in public this means, yes, a lot of lurking outside the doors of men's rooms, making friends with strange gents who just want to get their urination on.

Usually, honestly, when I explain that he is very firm that he is not going in the ladies' room, he has no trouble getting some help with the sink or the door or whatever, but everyone concerned is VERY clear on the thing where Mom is right on the other side of the door.