Is a tricky one - we have an IKEA bunk bed and it says should be 9! before on top bunk - it has rails so in my opinion this is very old. I think once you get over about 60-70cm above the ground it doesn't matter if is 1m or 1.5m off the ground is still probably not great to fall out of. I went into a bunk bed with rails at about 3-4 I think (and always had one from then on). We are going to play it by ear with CJ as to when she goes in top bunk but I am thinking similar age 3-4. From age 7 I never had rails, and never fell out until I was 18 years old!!! (I wasn't drunk, just slept the other way round to watch football and obviously rolled the wrong way) - I put my teeth through my lip on the fall but otherwise was unharmed. I remember my brother falling out too and he didn't even wake up!

I think falling out when sleeping is less of a problem then falling when trying to get in and out or playing in my opinion.