What's a good age?

I have a curious 5 year old who is always asking about pads, shaving, and being older.  I was raised by my very conservative grandmother, so I never had the puberty or sex talk...  Everything I learned as a pubescent tween came from television and movies.

I'm trying to raise my kids to feel comfortable enough to talk to me and their father when they've got any questions.  It's kind of difficult when I'm not sure what age is appropriate.  I don't want to scare or scar my kids ? Help!