When my son was first born, we co-slept, then at about 2 months he went into his crib. However, that only lasted till he was 6 months old, because I broke my leg at that point and he was still nursing in the night.
After that healed, he moved (Crib and all) into his own room, and for years slept in his own bed. Then when we moved, he was 3, and he occasionally wanted to sleep with mom. The summer before he was four, we had a guest for the summer, so he slept in my room full time (loaning his room out) and it was a little difficult to get him back into his own room. But we did.
Then (around January) my signifigant other at the time, made the big move to spending the night, and since then he has not wanted to sleep by himself. He starts out there most nights, but usually ends up in mom's bed.
He turns 5 next month, and I have explained that boys big enough for kindergarten need to sleep in their own beds. He claims there are monsters and he is scared. So we keep plugging away and on the mornings when he wakes up in his own bed, we make a pretty big deal about it.
So, there you go.