My plan was to get my son into a big boy bed once he could climb out of his crib. I did not want him getting hurt trying to get out. However, that day never came. When he was a little over three we moved into a house and decided to not set up the crib, and instead put him in a bed. He was old enough to understand the change, so I for him three was a good age.
We had tried putting him in a toddler bed at grandma's house when he turned 2 but he would just get out and get out and get out, it would take him hours to finally sleep. In the crib he would be asleep within minutes.
I think it really depends on you and your child and what works best for both of you.
at 9:53 AM on Apr. 13, 2011