We got to the point where DD would actually climb out of the cot, when it was set up at its lowest possible level, and that stressed me out imagining her landing on her head / neck.

So for us it was a safety first thing. She was about 18mths, but she's a real dare devil and loves to climb.

We started with a rail and placed one of those fold ot lounges under just in case. Then after a few days we got rid of the rail. It actually makes bedtime easier now as I can sit next to her while she is in bed and I read aloud to her!