It probably depends on your child. Mine is a right fidget and occasionally sleepwalks (as does his dad) so bunk/cabin beds are a no-no forever. If however you have a confident child who sleeps well, doesn't roll out of bed/fidget too much or sleepwalk then you are probably fine at 5, however watch out for the 3yo climbing up to play.
Son went into the bottom bunk aged 2, daughter in the top aged 5. Daughter has managed very well with getting herself out of bed and down the ladder with no injuries in the middle of the night - dream wees, etc.
Bunk bed lover, Really love your post. I shared a bunk bed with my brother and because I was the oldest brother I got the top bunk. We were aged 5 and 3 and there were no safety concerns. Unfortunately for my younger brother I occasionally peed the bed and the mattress was rather thin so he would get some drips rain down on him. C 'est la vie. Some great bunk beds below! www.mychildrensbeds.co.uk/