I think it was around 4-5 when we started telling the dds to wear clothes in public. In the house, they run around naked more or less freely (we don't like bare bottoms on the furniture, but otherwise we don't shame or make a big deal if they are naked). When they asked why, I'd say something like, "It's a social convention. In our culture, that's how people think people should dress, and I don't care enough about it to fight it." (They KNOW there are other issues that I care about a lot more, and am very willing to buck the trend.) That really is the long and short of it.
Thats really a question of hygeine more than social convention. Pinworms get spread that way.