J.B.
my daughter has a childs loft bed that sits about 4 feet off the ground (i think). she has been in it since she was 4 (shes 5 now). shes never fallen out and i remind her to move closer to the wall and not the edge. but for a higher teen type one i would get one of those safty bed rails to give her extra protection. though i would be more worried about her falling down the ladder in the dark then rolling off the bed.
Updated
my daughter has a childs loft bed that sits about 4 feet off the ground (i think). she has been in it since she was 4 (shes 5 now). shes never fallen out and i remind her to move closer to the wall and not the edge. but for a higher teen type one i would get one of those safty bed rails to give her extra protection. though i would be more worried about her falling down the ladder in the dark then rolling off the bed.