I've seen a 10 year old child sleep in one but I'd be hesitant to put an average American adult (200 pounds) in the either bunk. The canvas bunk sags slightly to the middle causing the person to roll there. Plus, the poles often scratched the walls when setting up.
Mine has a piano hinge on the back, runs the full length, and then two 1x or so braces that run along the sides. They must be tied into the framing. I happened to be up there, when I had my roof off, cutting back some paneling and was able to stand on the bunk and later had to roll around on it while stapling the new paneling. I am 190 lbs. I admit I did not feel completely comfortable standing on it but I lived to tell about it, he he. My unscientific measurement, not sure the bunk would take much more than that, but then again, it appears to be constructed to hold that and more. May come down more to the condition of bunk, screws, hinge, and framing it is tied into.