Stewie has been in my bed since he was 17 weeks. The first week (when he was 3lbs and 16 weeks old) he was in the hallway outside my room in his own bed. He cried one night and I caved and brought him into my bed with me.
Since then, I cannot sleep if he is not with me. I'm so spoiled -- and so is he.
I think once you let them into bed, both of you are ruined for life.