We took the side off the cot at about 20Months. I put a couple of thick blankets on the floor in case he fell out, but he never did. He was 3 before he got a proper bed though. Just for financial reasons.
Did he ever climb out? Or did you have any troubles with him going to sleep? DD goes to sleep great in her cot, I can put her in and leave her, but I'm anxious that she'll climb out and not sleep with no side.