Our 4 year old (has been in a full size single bed since about 15 months, because he could climb out his cot) he was walking from 9 months, but he was fine in the normal bed. He now sleeps in a normal sized single top bunk whilst our 21 month old sleeps in the bottom (he moved into the bunk bed when he was 17 months, again no problems). Our youngest started walking at 10 months. We don't have a guard on the door, but we live in a bungalow so it's not really a big worry for us. Plus I'm a real light sleeper and as soon as I hear their footsteps on the carpet I wake up.. LOL
I think it's just what they get used too, when I 1st moved them into their big beds they had a guardrail, infact our youngest still has one, but he probably doesn't need it.