My 7 year old sleeps on the top bunk, while we have a double/full size mattress directly on the floor underneath for my 12 year old son who is of a much larger size. He prefers to remain directly on the floor which is fine with us. It has saved a lot of space and is perfect in their small bedroom. 
The weight capacity for the top bunk is supposed to be 400 lbs, but I am deadly afraid to go up there and I'm half that weight. There is no way I'd go up there. It's a bit wobbly.
The ladder also hurts your feet! They should make each step flatter as opposed to round.