DD moved into a big bed at 17 months because we moved house and she's quite tall and had just about outgrown the cot. The first few nights she was unsettled but that could have been a combination of moving house and changing into a new bed. After that she started sleeping right through and has done so basically every night since (she's now almost 2 years old).
She started out with the matress on the floor and then at 21 months we moved the matress onto the frame with a side rail. Last week we took the side rail off altogether because she doesn't move much when she sleeps.