I think the appropriate time is when any involved family member becomes uncomfortable about it. That could be the dad, the mom, the kidster, the siblings. I've known families who were completely comfortable with nudity well into elementary years. If you are comfortable with it and the child isn't showing a desire for privacy yet, or unseemly interest, then nudity remains harmless.
Parents tend to feel about nudity what they were taught as children, so some parents might never want a child of the opposite sex to see any of their body parts after weaning. That's fine, too, especially if it is a matter of modesty, and not shame. Shame tends to express later in life as excessive interest, and many, many sexual addicts were given a big dose of sexual shame early on.
Even questions about body parts are usually nothing more than uncomplicated curiosity, and a great learning/teaching opportunity. Body parts have names and functions, which can be shared through the simplest possible descriptions.