my 11-year-old picked out this bed for the bookshelves and he loves it. It was easy enough to assemble my 16-year-old did it mostly by himself. We're using a 10" mattress, and it fits perfectly. The first shelf hits at about 13" from the floor, so I think a 12" mattress would have been a little too tall once blankets and pillows were added. We have it shoved all the way into a corner for better stability; I don't think it would feel very sturdy otherwise. Fingers crossed he doesn't get tired of sleeping basically on the ground for several years, but for now, it's perfect.