There has never been a stage in my life where I felt uncomfortable about the fact that my father and I would (and still do) occassionally see each other naked. I am still comfortable with functional, non-sexual nudity around my parents and other close family. My parents did an extremely good job of teaching me that functional, non-sexual nudity is no big deal by treating it like it was no big deal. I am attempting to do the same with my offspring.

Seeing as your question is about a child seeing the opposite sex parent naked (not about bathing with) then I would say there is never an age where it is inappropriate for a child to see either parent naked, taking into account that the nudity is both functional and non-sexual. (By functional I mean for a reason - showering / changing / etc..)