We're thinking about getting a cabin bed for DS1. He'll be 7 in October. My sister was going into a couple of independent furniture stores where we live and I asked her to see if they have any in their shops, how much they are, etc. When she asked about cabin beds, she has been told in each shop that they do not recommend them and prefer not to sell them. In one shop she was told that they are a "death trap" and that he certainly wouldn't put his children in one!
we use a mid-sleeper for dd (4.8 years)and she has never had any problems. My niece (9) uses a high sleeper bed and also hasnt had a problem. A 7 year old should definitely be ok in one (the high sleepers arent recommended before the age of 6). The cabin beds dont seem to have much to keep lo's from falling out so maybe that is why they said they were a 'death trap' as they are a bit further off the ground than a normal bed so there is further to fall. The mid and high sleepers have raised edges which helps stop accidental falling out in the night.
Ds (5)bhas a mid sleeper and has never had any problems. I have never fallen out of bed in my whole life - not sure why ds is suddenly likely to. Also, the bed does have rails on so you have to roll over the rails!