18 mo. Kind of had to as he had fallen out once, and was waking himself up when he rolled over and hit his arms against the bars (sleeping to ds is a martial art, frankly, he spins like Jackie Chan). But also because I foolishly thought he would entertain himself in the mornings if he could get to his toys!
HAHAHAHAHAHHAHAHAHAHAHAHAHAHAHH
It was much too early and we spent a nightmare 5 months when he literally would not spend a moment in the bed when he was awake. Possibly cot sides would have helped but I don't believe so myself.
If I were you, I would stick a mattress by the cot to catch her if she falls, and spin it out as long as possible. But I know I wouldn't have been able to do that myself. And some children apparently do stay in their beds much more than ds.