E.,
I think my son was about 2 when we moved him out of the crib, and that was because he had outgrown it. We started with the box springs and mattress on the floor, then we put his bed together and added a fold down side rail after x amount of time. When he was 3 1/2 we moved into a bigger house and he told us he didn't want the side rail anymore. His transition out of a crib was easy. He's now 5 1/2 and would rather play with his toys for half the night, then get up with the sun; but that's a different can of worms.
My daughter just turned 2 in March and we moved her out of the crib about a month ago when I noticed some of the screws/bolts were stripping out and it was no longer safe for her. We thought we could borrow a box springs and mattress set from my dad until we got her her own set. No mattress, just a box springs was there. She hated the crib mattress on the floor. She flat refused to let us put the crib mattress on the box springs on the floor. She got too cold sleeping on just the box springs. We finally bought her a bed frame and put the box springs in the frame and added the side rail. The sides of the frame bowed out, the slats fell through the bottom and the box springs followed. We took the side rail away. She figured out how to climb down. She fell out of bed multiple times. Super Nanny's back to bed routine helped. She wandered the house after I went to bed. Three days later she learned to climb over the gate placed in her doorway, even raised about 8 inches off the floor. We bought a mattress from a discount store and put the side rail up that her brother had used. She sleeps better and better every night.
My point? Every child is different and you won't know she's ready until you try. Many bits of advice I've heard is that the child is ready for a twin/toddler bed when they start climbing out of the crib. Neither of my kids did that. My daughter is the monkey in the family and will fearlessly climb the 8 foot bookshelf if we let her. Never once climbed out of the crib, even when I would accidentally leave the side down all night. She'd climb up and stand on the lowered side while calling for me, but never climbed fully out on her own. (Don't get me started on her big brother helping her climb in and out. Again, another can of worms)
Hope this rambling helps,
Melissa
Oh, and maybe let her choose which bed she would like to sleep in, if there is room to have both the crib and toddler bed in there at the same time. My house doesn't have the space, so it was ok, we're switching now.