DD was around 18 months old. We did it simply because she refused to sleep in her cot. At first she slept on a mattress on the floor because I didnt want to buy a bed if she wouldn't sleep on it. After a few weeks we got the bed. She didnt want to sleep in the bed just the mattress but now sleeps in her bed.

We have yet to master her going to sleep without one of us in there though Cant win them all I guess