We moved DS inot a big boy bed when he was 14 months, we took the rail off the cot at 13 months as he started climbing and a distant friend of ours lost her child when they climbed the rail and got their bib stuck and choked to death, so I would rather DS fall the 35cm to the floor than 1metre from the rail, or anything worse.

We got a race car bed of ebay 2nd hand for $50 and its great as it has the little sides on it so it stops him rolling out. Haven't regretted it once, he's so happy in it and we have our minds at peace!