Hi R.!
My son was sleeping in a twin bed(not just toddler!) at 15 months old. He's a HUGE tosser and turner, and when in his crib, he would get a foot stuck, bang his head, or get his arm twisted somehow. Thus, he NEVER slept through the night. I had my twin bed in his room (no where else to put it!). So I bought a rail thing and started placing him in that bed for naps. And he slept wonderfully. A week later, I put him in at night, and like magic, he started sleeping through the night.
I asked my pediatrician about it, and he said as long as my son could climb in and out on his own, he should be safe. My son was always big for his age and far ahead with gross motor. So climbing up and down from the bed was no problem. I don't think there's a "best" time to transition. I think it's when your child is ready or being in the crib is no longer safe; i.e. climbing out. Good luck!