Bunk Beds

How old should a child be to sleep on a bunk bed.  I remember going on a bunk bed a few times sleeping over friends houses and they were kind of scary to me and I was around 10 years old and I remember it like it was yesterday.  My sister wanted to get her son one and I just think that he should at least be 6 years old to sleep in a bunk bed but that is just me.