I was wondering if it's illegal to take pictures of a baby while he's naked, for example. I'm mostly asking for babies, but it would be nice to know what age is ok to do it.
Yes, it's legal. To be illegal, the photographs would have to be obscene or pornographic. Nude people, ordinarily photographed, doing non-sexual, non-excretory things are neither. However, I would strongly advise caution, particularly if you plan to display or share these photographs. There are definitely cases of people who have been charged with sexual exploitation of a minor for sharing nude photographs of their children.