This past year I started watching them with my 3-year old (he'll be 4 in July). He knows the Star Wars characters better than any kid I've met, so I wanted to let him see some of the movies (though my wife wanted him to be a bit older). We started with Ep IV and waited a bit until we watched another one. He seemed to have some bad dreams. But I've shown him IV, V, VI & I. In that order. The prequels are a little more violent, it seems, so I'm a little slower in showing him those. It'll be a while until he sees Ep. III since it's PG-13 and the others are all PG. He's not ready to be exposed to that yet.
if he were older, I'd probably shown him to them in 1-6 order. But for as young as he is, the OT is a little more kid friendly (Lucas tried to make the PT with more kid appeal, but it's just not as kid friendly as the OT).