at 18 months I moved my son from crib to a twin mattress on the floor, putting the rail up so he gets used to it. And if he fell out, he would fall 3 inches.... LOL!
Was so easy for him to get in and out and gave him independence and confidence, as he was now in a big boy bed, with big boy bedding (Cars and trucks as apposed to Nemo and other sea creatures)
He loved it and within a few months we were ready for the box spring. We put his mattress where his crib used to be so he was still familiar with his sleepy space.
I moved his bed two months ago closer to the window so he could get more light this summer.
The good thing about having a twin bed is they love the space, and also you can lay beside them and read to them or just have a nice chat and giggles.
Good luck Johanna