As someone who is about to be a first time parent this will be a consideration that I'll have to face as well... that said the original question here was AT WHAT AGE to introduce kids to SW and not WHICH MOVIES to choose to watch. The latter is the choice of the parent(s) just as it is with every other type of media or entertainment choice.

So... I don't see anything in the OT that is potentially harmful to a child's mind from about age 4 and up. Then again I think that might be a bit too early to get the full impact of the adventure one is seeing. I saw 'SW' on my 6th birthday and (as evidenced by my presence here) I got hooked. And the *gasp* 'slower' pacing of the OT might help kids who aren't just looking for video game-ish loud flashy images and actually want to get a feel for characters. 'Phantom Menace' would be the best, imho, of the PT for a younger child (4-6) to see. 'AOTC' and 'ROTS' probably have moments a bit too dark but then again Luke decapitating Vader in 'ESB' is a bit dark too.

So I guess stick with the more kid-friendly films like 'SW', 'ROTJ' and 'PM' to start with... baby steps into a bigger world.