K.B.
I transitioned at a younger age with my son. He was younger than 18 months. (Can't remember if it was 15 or 16 months old). He just cried in his cribe, and wanted to sleep on the floor. I put him in a big boy bed and he did awesome. (Until he got sick and we got into bad habits). I found it easier at this time b/c he wasn't attached to the bed, and it was what he wanted. Also, we had a grow with me bed, so it was simply taking the side off, the rest of the bed stayed the same. On a side note: he was in day care and sleeping on a cot, and he slept in a toddler bed with a small rail from a year old at his grandparents house. He knew from a very young age to just stay in bed. As long as there was a rail he never ever tried to get out, even though he could have easily done so.
Some kids transition better than others. I wish you the best of luck!