We put Emily in a single bed at 16 months. We needed to reclaim her cot and didn't want to do it too close to the arrival of the new baby.
She was fine, no problems at all really.
I think as long as they can get in and out of bed themselves it's all good. If they can't you might find yourself having to go in and help them alot more than you'd like to be. You could put a mattress on the floor next to the bed - this will help give them some more height to get up as well as cushioning should she fall.