C.K.
We have never used a toddler-size bed. Both my girls went directly to an adult twin bed from the crib. My first didn't transition until she was 4 because she had special needs and had some developmental delays. It was a good time because I was pregnant and we needed to transition her anyway before her sister came. Then, I read an article in the newspaper one Sunday quoting Dr. T. Berry Brazelton, a well known pediatrician who has written several books and used to have his own tv show. Dr. Brazelton recommended trying to wait until your child is 3 or 4 years old, if you can. It will eliminate any insecurities, fears, etc., the toddler may have making such a transition. He said the only time you should transition them sooner is if they begin to climb out of the crib or if you have another baby on the way that will need to use the crib. So, we didn't transition our youngest until she was 3. Both transitions were very successful. Partially due to the fact that before moving them to the twin at night, we let them take naps in it during the day just to get used to the idea.