I think it depends alot on your childs abilities and development. I don't think there she be a certain age, because i is different with each child. Is she ready?
I didn't take my son out of the crib until he was almost two and a half. He never really climbed out, and this way I didn't have to worry about what he was doing while I was sleeping. (He is an escape artist) I took him out because he got too big and we were expecting another one. When I took him out he went straight to a twin bed. We padded the floor with blankets, but he never really needed them. He falls out a few times a year.
My daughter is over two and she is still in her crib. She doesn't climb out and I don't have to worry about her while I'm sleeping. (Escape artist also)
I suggest buying a twin bed and getting the side rails that go on them. This way might even save you money or time. Or you could do what my sister did with her four girls. She started by putting the crib mattress on the floor. This allowed the child to get used to a regular bed without haveing to fall so far and hard. Then she put a twin mattress on the floor, no box spring. This allowed the child to get use to the size of the bed and to learn where the edges were. Then put the mattress on the box spring/frame. If you are still concerned pad the floor up with pillows and blankets.
Otherwise, if she is starting to climb out, put her in the bed. Falling from an attempt at climbing could cause more injury than falling off the bed, her legs/feet could get caught in a rail. Children are made to be durable because they do fall/bump alot. Just pad the floor by her bed with pilows and blankets, clear the area near her bed of all toys, books, etc. Make sure ther is nothing that could hurt her if she fell on it. Then she should be okay.