Really it depends on the subject of the forum. It's the internet, there are forums to discuss ANY subject you might consider tabboo no matter how strange they are! Anything that might offend a minor is general a good idea to avoid unless it's an adult based site.

First of all, I do not worry about the age of users on my forums. Parents need to parent. It's not my place or my problem to child-proof the universe for someone else's kid. That has become a big problem in the world today. Parents need to deal with these issues themselves.
That said, none of my forums have explicit sexual content or discussions. It just never comes up. However there is profanity.

I have found over the years that the more you moderate, the less active the forums are. I don't worry about offending people. If they don't like it, they can go elsewhere. If they don't want their child there, they can make the kid stay away.